How to Separate Natural from Synthetic Ametrine using Conventional Equipment

How to Separate Natural from Synthetic Ametrine using Conventional Equipment

Guy Lalous ACAM EG summarises the discovery of features that could be used to distinguish between natural and synthetic ametrine from The Journal of Gemmology; from the orientation of growth striations to the interference patterns caused by twinning.

What about natural ametrine?

Ametrine is a bicolored quartz variety that contains both amethyst and citrine zones in the same crystal. The only significant source of natural ametrine is eastern Bolivia’s Anahi mine, where it occurs in veins in a dolomitic limestone. The amethyst-citrine bicoloration results from quartz precipitation at very specific geochemical conditions, temperatures, and growth rates. The combination of amethyst and citrine colours in natural ametrine from the Anahí mine has been attributed to colour zoning that differentiates rhombohedral r (violet) and z (yellow) growth sectors.

The colour of iron-bearing quartz depends on the valence state of the iron. The citrine colour in Bolivian ametrine appears to come from the incorporation of very small aggregates of Fe3+. The amethyst colour develops in two steps. First, individual Fe3+ ions replace Si4+ ions in the quartz structure. To develop the amethyst colour, the crystal must be exposed to ionizing radiation to oxidize the iron in the 4+ state.

Shown in this composite photo are three custom-faceted natural ametrines: a 19.87 ct round StarBrite cut, a 20.35 ct cushion ZigZag cut and a 13.69 ct square StarBrite cut. Courtesy of John Dyer Gems, Edina, Minnesota, USA; photos by Ozzie Campos.

What about FTIR?

FTIR is a technique that measures absorptions within the infrared part of the electromagnetic spectrum. In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample due to vibrations of molecules in the crystal structure and some of it is transmitted. The resulting spectrum represents a molecular fingerprint of the sample. Infrared spectrometry is very useful to detect impregnations in gemstones (polymers, oils and resin), heat treatment in corundum and to distinguish certain natural and synthetic gem materials. Full width at half maximum is the width of the spectrum curve measured between those points on the y-axis, which are half the maximum amplitude.

What about EDXRF?

X-Ray fluorescence analysis using ED-XRF spectrometers is a commonly used technique for the identification and quantification of elements in a substance.

Beginning in 1994, Russian gem-quality synthetic ametrine entered the market. Synthetic ametrine can be identified by employing advanced techniques, such as EDXRF chemical analysis, and IR spectra. High-resolution (0.5 cm-1) FTIR analysis has shown that a band at 3595 cm-1 is present in the vast majority of natural amethyst. If the 3595 cm-1 band occurs in synthetic amethyst, it has a much larger FWHM (Full width at half maximum) value than in natural specimens. EDXRF chemical analyses revealed higher concentrations of K, Mn, Fe and Zn than in natural ametrine.

What is a conoscope?

The conoscope is a polariscope accessory tool. It is a strongly converging, strain-free glass sphere. When a gemstone is positioned between two crossed polarizers, interference colors that are centered in the specimen will be witnessed with the conoscope when the optic axis is exactly perpendicular to the polarizers.

Previous studies focused on the possibility to separate natural from synthetic ametrine using the refractometer and the polariscope. Quartz is a uniaxial mineral with two unique refractive indexes along its three crystallographic axes. The unique axis is the optic axis. The amethyst-citrine colour boundary in natural ametrine is oriented roughly parallel to the optic axis; in synthetic stones, the boundary is oriented at an oblique angle to the optic axis. The gemmologist needs only to find the direction of the optic axis to determine whether an ametrine is natural or synthetic.

The optic axis in a uniaxial gemstone can be found with a polariscope that has a conoscope lens and, on occasion, with a refractometer. The direction of the optic axis cannot be obtained by refractometer readings for samples cut with their table at random orientation to the optic axis and some difficulties may arise with samples displaying complex colour zoning or twinning.

In this article, the authors explain the possibilities for separating natural from synthetic ametrine by microscopic examination. The immersion microscope was used to look for twinning features, to establish the orientation of the violet/yellow colour boundaries and the direction of growth striations relative to these boundaries, and to observe any characteristic inclusions.

Faceted natural ametrine gemstones from Bolivia typically display only two colour zones, as seen here viewed toward the table facets (top) and toward the pavilions of the same samples (bottom). The stones weigh from 2.45 to 7.45 ct (upper left, 11.7 × 10.8 mm). Photos by K. Schmetzer.

 What are Brewster fringes?

Amethyst from worldwide localities is commonly Brazil-law twinned, which is an intergrowth of right- and left-handed quartz. Such twinning is evidenced only by examination under polarized light. It results in sectors which, when viewed perpendicular to the c-axis, show symmetrical trigonal patterns of dark bands known as Brewster’s fringes. In Bolivian ametrine, these fringes are found only in the alternating amethyst sectors, and not in the citrine sectors.

Between crossed polarizers, the samples show interference patterns (Brewster fringes) that indicate Brazil-law polysynthetic twinning of the violet r growth sectors. Photomicrographs by K. Schmetzer, in immersion.
Optical FeatureNaturalSynthetic
Twinning

Violet growth sectors are intensely twinned on the Brazil law, showing various forms of Brewster fringes with crossed polarizers;
yellow growth sectors are not polysynthetically twinned.

Violet and yellow growth sectors are primarily untwined; small areas within the violet growth
sectors may be twinned on the Dauphiné and/or the Brazil law.
Violet/yellow boundaries Mostly parallel to the c-axis or only slightly inclined to the c-axis (up to about 10°).

Inclined between 20° and 38° to the c-axis.
Growth striations Violet growth sectors: inclined at about 67° or 38° to the violet/ yellow boundary; yellow growth sectors: none observed.

Violet growth sectors: parallel or almost parallel to the violet/ yellow boundary, mostly inclined
at angles between 0° and 8°, with a maximum inclination of 18°; yellow growth sectors: very
weak striations parallel to the basal face.
Fluid inclusions

Rare fluid inclusions, occasionally reflecting the polysynthetic twin pattern of the violet growth zones.

Rare two-phase (liquid and gas) inclusions elongated parallel to the c-axis.
Table showing diagnostic features of natural and synthetic ametrine using immersion microscopy.

The microscopic procedure for identifying these key features can be summarized as follows. The examination of a faceted sample of unknown origin should begin by orienting the dominant colour boundary perpendicular to the rotation axis of the sample holder. If the stone is natural, the typical interference pattern with Brewster fringes will be revealed upon rotation of the sample.

Furthermore, growth striations inclined at relatively large angles to the colour boundary will be observed in the violet portion of the stone after a rotation of about 40° versus the c-axis. If the sample is synthetic, rotating the sample generally will not bring the optic axis into view, and violet growth striations parallel or at a small angle to the violet/yellow colour boundary frequently will be present. It is possible to find the optic axis in a synthetic sample by moving it to other orientations within the sample holder, in which case an untwined interference figure normally will be seen.

In natural ametrine, the colour boundary between the violet r and yellow z growth zones more-or-less follows a prismatic m crystal face but is not exactly planar. In addition, growth striations are present in the violet r sectors, and they are parallel to an external r face and inclined to the violet/yellow boundary. The angle between the growth striations and the colour boundary measures approximately (A) 67° or (B) 38°. Photomicrographs by K. Schmetzer, in immersion.

Separating synthetic ametrine from its natural counterpart using conventional gem lab equipment is possible, provided that the gemmologist has a good understanding of the morphology and optical mineralogy of both natural and synthetic material. The authors insist to use immersion for microscopic observations as the various patterns or structures observed without are of less diagnostic value.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled 'Distinction of Natural and Synthetic Ametrine by Microscopic Examination - A Practical Approach' by Karl Schmetzer 2017/Volume 35/ No. 6 pp. 506-529

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

Cover image: Crystal clusters that occupy the storage room of the company Minerales y Metales del Oriente in Santa Cruz, Bolivia. Only small portions of the crystals are of facetable quality. Photo taken in 1997; courtesy of Udo Reimann.

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An Update on Identification Features of Treated Baltic Amber

An Update on Identification Features of Treated Baltic Amber

Guy Lalous ACAM EG explores a detailed characterization of Baltic amber samples treated in experiments by Wang et al.(2014) in The Journal of Gemmology, whilst discussing the criteria for identifying heat-treated amber using standard gemmological instruments, FTIR and Raman spectroscopy.

Amber is an organic gem. Organic gems are the products of living or once-living organisms and biological processes. The fossilisation process of amber involves a progressive oxidation, where the original organic compounds gains oxygen, and polymerisation, which is an addition reaction where two or more molecules join together. This process produces oxygenated hydrocarbons, which are organic compounds made of oxygen, carbon and hydrogen atoms.

What about “Beeswax amber”?

Beeswax amber is sub-translucent or opaque due to abundant microscopic bubbles.


(a) Surface features of aged beeswax amber may include oxidation cracks, as shown here on sample JD-2; note the unoxidized centrelines and only minor 'bleeding' of adjacent colour. (b) By contrast, cracks formed in beeswax amber during natural weatheing have dark centrelines with more extensive 'bleeding' of colour, as seen here on untreated amber JB-3. Photomicrographs by Y.Wang; magnified 20x.

What about “Sun spangles”?

In heat treated amber, discoidal stress fractures are produced by expansion and decrepitation after the pressure equilibrium of the bubbles within the amber has been abruptly broken. Those fractures are referred to as “Sun spangles”. The presence of large and numerous sun spangles within amber provide immediate evidence of heat treatment.

What about treatments?

The main purposes of amber heat treatment are to improve or alter the colour, enhance the clarity and produce inclusions that have an appealing visual effect (“Sun spangles”). Methods include clarifying, baking (oxidation), decrepitating and ‘beeswax ageing’.

This paper provides a detailed characterization of Baltic amber samples that have been treated in experiments by Wang et al.(2014). Standard gemmological instruments were used as well as FTIR and Raman spectroscopy to document changes in the physical, optical and spectroscopic properties of the samples before and after treatment. The formation mechanisms of the features seen in heat treated amber are discussed, and criteria for identifying heat-treated amber are presented.


'Sun spangles', or discoidal stress fractures, were exhibited by heat-treated amber samples that underwent decrepitation. They are shown here within: (a) golden fire amber JE-5 (magnified 15x) and (b) red fire amber JC-3 (20x; the red colour of this particular discoid fracture is partially obscured by yellow reflections from the surrounding amber). Photomicrographs by Y.Wang.

What about the “Baltic shoulder”?

Infra-red spectroscopy is the most effective scientific method for identifying fossil resins. With this technique broad absorptions will be witnessed in Baltic amber in the 1260-1160 cm-1 range. Those are assigned to C-O stretching vibration. These features known as “Baltic shoulder” are specific to Baltic amber and are related to the presence of succinic acid. Baltic amber, also called succinate, contains 3-8% succinic acid.

The gemmological and spectroscopic features of untreated versus treated amber are listed in the table below:

Gemmological & Spectroscopic Features Untreated Baltic Amber Treated Baltic Amber
Colour & Clarity

Yellow or light yellow beeswax

Opaque or translucent

Golden yellow, red or dark red

Transparent, some with transparent surface & opaque interior
Refractive Index 1.54

Clarified golden: 1.54 – 1.56

Oxidized red: 1.55 – 1.58
Long-wave UV Fluorescence Moderate-to-strong yellow to yellowish white

Clarified golden: weak-to-moderate dull yellow or yellowish white

Baked red: inert or weak dull yellow
Internal Features  

Sun spangles

Red flow striations
Surface Features with no re-polishing

Oxidation cracks: dark centrelines – extensive “bleeding” of colour

Oxidation cracks: narrow- minor “bleeding” of colour

Septarian cracks: irregular networks of micro-cracks showing mosaic-like appearance

Wavy surface ripples  
FTIR Spectra

Strong absorption bands at 2932 and 2867 cm-1

Absorptions at 1732 and 1702cm-1

Moderately strong absorptions at 1452 and 1378cm-1

Broad absorptions into the 1260-1160cm-1 range (“Baltic Shoulder”)

Absorptions 1645 and 888 cm-1

Decreased intensity of 2932 and 2867 cm-1 (*)

Increased intensity 1732 and 1702cm-1 (*)

Increased intensity of 1260- 1160 cm-1 (“Baltic Shoulder”) (*)

Decay to extinction of 1645 and 888 cm-1 (*)
Raman Spectra Peaks at 2932 and 2867 and 1645 and 1444 cm-1 Minor changes: The absorption intensity at 1645 cm-1 dwindles and the intensity at 1444 cm-1 increases gradually

(*) Progressively with clarification and oxidation


Untreated amber specimens from Kaliningrad Russia, were sliced into multiple pieces for heating experiements. Each sample number is shown with the total weight of all the slices. Photos by Y.Wang


Pictured here are the same samples in the pictured above following various treatment processes, as described in the text and in the table above. Photos by Y.Wang

What about the variations of the FTIR features after heat treatment?

A decrease of the absorption of the major band at 2932 cm-1 suggests that the saturated C-H bond was broken down by heating. An increase in intensity of the absorption at 1732 cm-1 suggests that oxygen involvement enables a higher concentration of the C=O functional group. The extinction of the weak absorptions at 1645 and 888 ccm-1 corresponds with the breaking of the unsaturated C=C double bond of the exocyclic methylene group. Heat treatment leads to fewer saturated C-H bonds and unsaturated C=C double bonds in amber, in correlation with more oxygen-bearing functional groups and higher degree of polymerization.

What about the variations of the Raman features after heat treatment?

FT-Raman spectra indicate that the number of saturated C-H bonds (1444 cm-1) consumed by oxidation during amber heat treatment is greater than that of unsaturated C=C double bonds (1645 cm-1) consumed during the process, and thus higher intensity ratio (I : 1645 cm-1/I = 1444 cm-1) indicate a greater degree of oxidation.


Representative FT-Raman spectra are shown for sample groups JA and JD, both before and after heat treatment.

What are the conclusions reached for the spectral intensity ratios after heat treatment?

FTIR: An intensity ratio of ≤ 1.54 for the 2932 and 1732 cm-1 bands is indicative of clarified amber, while ≤ 0.50 correlates to baked amber, the range of ~ 1.5 – 1.9 is not considered diagnostic. Raman: The minor changes in intensity ratios are not considered as being diagnostic for identifying heat treatment.

The treatments of amber are increasing in number and complexity. Baltic amber is the most versatile when it comes to treatments.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Gemmological and Spectroscopic Features of Untreated vs. Heated Amber’ by Yamei Wang, Mingxing Yang, Shufang Nie and Fen Liu 2017/Volume 35/ No. 6 pp. 530-542

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

Cover image: Golden amber pendant (upper left, 36.65 g), a bicoloured fire amber pendant (upper right, 22.59 g), a red amber necklace (96.27 g) and an aged beeswax bracelet (45.31 g). Photo by Y.Wang.

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Inclusions Acting as Geological Fingerprints in Yellow Danburite from Vietnam

Inclusions Acting as Geological Fingerprints in Yellow Danburite from Vietnam

Guy Lalous ACAM EG summarises an article on inclusions in yellow danburite from Luc Yen, Vietnam, by Le Thi-Thu Huong, Kurt Krenn and Christoph Hauzenberger, originally published in Gem-A's The Journal of Gemmology. His report also examines optical characterisation of danburite and micro-Raman spectroscopy.

Danburite is a calcium borosilicate, CaB2Si2O8, belonging to the orthorhombic crystal system. It is genetically associated with rocks of magmatic (granite pegmatite), metasomatic (skarn) and sedimentary (evaporite) origins.


This 31.6g danburite specimen from the Bai Cat alluvial deposit in the Luc Yen mining area shows eye-visible hollow tubes and overlapping 'fingerprint' inclusions. Photo by L.T.-T. Huong.

What about metamorphism, marble and pegmatites?

When rocks change because of an increase in the pressure and/or temperature of their surroundings, it is called metamorphism. Marble is a metamorphic rock primarily composed of calcite, formed when limestone is subjected to heat and pressure. Pegmatites are igneous rocks. Igneous rocks are formed through the cooling and solidification of magmas. Pegmatites contain extremely large crystals and rare minerals. Most pegmatites have a composition with abundant quartz, mica and feldspar.

Černy's scheme (1991) is the most widely used classification of pegmatites. It combines depth of emplacement, metamorphic grade and minor element content. It is divided in 4 main categories. The abyssal (high grade, high to low pressure), Muscovite (high pressure, lower temperature), Rare-Element (low temperature and pressure), and Miarolitic (shallow level).

The Rare-Element Classes are subdivided based on composition into LCT type (Lithium, Cesium, and Tantalum enrichment) and NYF type (Niobium, Yttrium, and Fluorine enrichment). The Rare-Element Class if further subdivided into types and subtypes according to mineralogical/geochemical characteristics. Many pegmatites fall nicely into these categories but some of the Madagascan pegmatites are virtually unique and don't fit into these categories.

Luc Yen is a mountainous district located in the north of the Yen Bai providence in Vietnam. Ruby, sapphire and spinel have been recovered from primary and secondary deposits since 1987 in the Luc Yen area. The geology of Luc Yen is dominated by metamorphic rocks - mainly granulitic gneiss, mica schist, and marble - that are locally intruded by granite and pegmatitic dykes.

This paper is an updated description of the inclusions in danburite from Luc Yen, which were characterised by optical means and by micro-Raman spectroscopy. The original source rock type for this alluvial danburite is then proposed, according to information provided by the study of the inclusions.

Internal features in Vietnamese danburite consist of fingerprints, hollow tubes, and two-phase and multiphase fluid inclusions. The two-phase inclusions were typically composed of a liquid and a vapour bubble that showed various proportions, suggesting heterogeneous entrapment of the dominant fluid during crystal growth.

Most of the multiphase inclusions contained several crystals, a liquid phase and a vapour bubble. The crystals in the multiphase inclusions typically formed colourless euhedral pseudohexagonal plates; some of them displayed interference colours.


(a) Primary  multiphase inclusions occur singly or are arranged along trails, both parallel and perpendicular to the c-axis of the host danburite crystal. (b) The CO2 vapour bubbles in the two-phase inclusions vary in size, and in some cases only a thin layer of liquid is present along the inclusion walls. (c) Sassolite crystals in the multiphase inclusions appear as colourless, pseudohexagonal plates - sometimes displaying interference colours - with more-or-less perfect crystal faces. (d) Calcite crystals are occasionally associated with the sassolite plates in the multiphase inclusions. (e) This fluid inclusion contains multiple sassolite crystals accompanying two CO2 vapour bubbles and a liquid (H2O). Photomicrographs by K.Krenn.

What about Raman-spectroscopy?

In Raman spectroscopy the studied sample is illuminated with a monochromatic laser (single wavelength), the light is scattered by the sample. Light scattered from the sample is due to either elastic collisions of the light with the sample's molecules (Rayleigh scatter) or inelastic collisions (Raman scatter). Raman scattered light returns from the sample at different frequencies that are proportional to the vibrational frequencies of the bonds of the molecules in the sample. The Raman scattering from every molecule is different as the bonds for every molecule are different.

A Raman spectral 'fingerprint' can be generated. A database of reference spectra is necessary as the identification of a mineral by Raman spectroscopy is a comparative method. Raman spectrometry is useful to identify gems, inclusions and filling substances in gemstones.

What about sassolite?

Sassolite H3BO3, crystalline boric acid has been described for the first time as an inclusion in gas-liquid inclusions in minerals from the pegmatite veins Mika and Amazonitovaya in the Kukurt gemstone district in Central Pamir in 2000 (S.Z. Smirnov et al.). The crystals were rounded, tabular and less frequently idiomorphic with low refractive indices and high birefringence. The Raman spectrum of sassolite revealed an intense line near 880 cm-1 and a weaker one at 449 cm-1. The data obtained allowed reconstructing the conditions of formation of both granite pegmatites and hydrothermal systems where boron actively participated in mineral formation. Sassolite is a characteristic component of fluid inclusions in minerals from the majority of tourmaline-bearing and topaz-beryl miarolitic pegmatites.

Raman spectroscopy of the multiphase inclusions in the danburite samples revealed that most of the crystal inclusions were sassolite with occasional crystals of calcite. The sassolite showed two distinct bands at 500 and 880 cm-1 and two additional bands at 3165 and 3247 cm-1. The 500 and 880 cm-1 bands are assigned to vsB[3] -O species, where B[3] -O denotes three-coordinated boron. The calcite was characterised by two strong bands at 1088 and 283 cm-1 and a less-intense band at 714 cm-1.

The doublet at 270- 300 cm-1 in the Raman spectrum of the calcite might result from a combination of the intense calcite band at 283 cm-1 with the two nearby danburite bands (~281 and 296 cm-1). The liquid and vapour phases were identified by Raman spectroscopy as H2O and CO2, respectively. The spectrum of CO2 in the danburite fluid inclusions shows two diad peaks positioned at 1285.1 and 1388.3 cm-1. The diad split (known as the Fermi doublet) corresponds to low-density values, which points to a granitic pegmatite source rock for the danburite.


(a) Micro-Raman spectroscopy of the danburite fluid inclusions in the 150-1500 cm-1 range shows the presence of sassolite, calcite and CO2 vapour. The black trace shows only the bands of the host danburite. Labelled peaks are from the inclusion phases present. (b) Raman spectroscopy of the fluid inclusions in the 1500-3000 cm-1 range shows no bands from additional gas phases (e.g. N2 and/or CH4), but only luminescence signals of the host. (c) In the 3000-3800 cm-1 range, Raman spectroscopy of the fluid inclusions shows additional bands for sassolite (at 3165 and 3247 cm-1) and characteristic bands of water.

The various proportions of a carbonic vapour phase (CO2) compared to a liquid phase (H2O) indicate a heterogeneous entrapment of the fluid inclusions. This suggests that the associated sassolite and calcite precipitated as a result of decreasing temperature through hydration reactions with the host danburite. The presence of sassolite together with low-density H2O - CO2 fluid inclusions indicates the Luc Yen danburite originated from a granitic pegmatite source rock.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Sassolite- and CO2-H2O-bearing Fluid Inclusions in Yellow Danburite from Luc Yeb, Vietnam’ by Le Thi-Thu Huong, Kurt Krenn and Christoph Hauzenberger 2017/Volume 35/ No. 6 pp. 544-549

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

Cover image: Hollow tubes in a 31.6g danburite specimen from the Bai Cat alluvial deposit in the Luc Yen mining area. Photomicrograph by L.T-T. Huong; field of view 2.5 cm.

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Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

Read more

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

Read more

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

Read more

Tanzanite: The Contemporary December Birthstone

Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

Read more

Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

Read more

Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Read more

Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

Read more

Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

Read more

 

Additional Info

Read more...

The Source of Garnets Found at The Arikamedu Archaeological Site in South India

The Source of Garnets Found at The Arikamedu Archaeological Site in South India

Guy Lalous ends the year with his final Journal Digest of 2017 by exploring the chemical and mineralogical characterization of garnets found at the Arikamedu archaeological site in South India and their linkage to the rough material sourced from the Garibpet Deposit, roughly located 640 km away in Telangana State, east of the city of Hyderabad, India.

What about silicates and nesosilicates?

The vast majority of the minerals that make up the rocks of Earth's crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and garnets. The building block of all of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom. Garnets are nesosilicates. This subclass includes all silicates where the (SiO4) tetrahedrons are unbounded to other tetrahedrons.

What about garnets?

Garnets are a set of closely related minerals that form a group, resulting in gemstones in almost every colour. All garnets have essentially the same crystal structure, but they vary in chemical composition and properties. Many garnets are chemical mixtures of two or more garnet species, they are found throughout the world in metamorphic, igneous and sedimentary rocks. They have been grouped according to their composition in two groups. The ones that contain A1 in the B position in the formula are widely called pyralspites and the ones with Ca in the A position are ugrandites. These names are derived from the first letters of the single minerals in these groups; pyrope, almandine and spessartine make up the pyralspite and uvarovite, grossular and andradite are the members of the ugrandite group.

Garnets are isotropic and figuring out how each one of them fits into the six main mineral species and their mixtures can be a serious challenge. The gemmologist will need an accurate refractive index, specific gravity and UV-VIS spectrum to come to the right conclusion.


Table of identification of the main garnet species.

What about Arikamedu?

Arikamedu has initially been portrayed as a Roman settlement. Modern theories describe Arikamedu as an important Indian trading centre and harbour, connecting the east coast of India with the western world from the 1st century BC to the 7th century AD. Arikamedu served as one of the main bead-producing localities in India. The unearthing of several thousand stone and glass beads during the archaeological excavations attests to this fact. The glass is rich in potassium oxide K2O (Harder 1993). Some of the beads collected are cobalt glass with following gemmological properties: R.I.=1.52 spot, gas bubbles, swirl marks and chalky fluorescence (Jayshree Panjikar, Pangem Testing Laboratory, Pune, India). Garnets were the second-most prevalent among the stone beads after the quartz variety. Bead production remained on-going in the region for centuries and was only abandoned in the early 17th century.

The current study presents for the first time a thorough chemical and mineralogical characterization of garnets found at the Arikamedu archaeological site in southern India, using high-quality major- and trace-element data in conjunction with detailed inclusion studies. The authors then demonstrate a remarkable correlation with recently mined garnets from Garibpet in Telangana State, India - approximately 640km away of 760km distant by road - as the source of origin.

The Kothagudem-Garibpet area is located in the Vinjamuru domain of the Khammam schist belt and consists of Paleoproterozoic moderate-grade (and partly migmatitized) metasediments and metavolcanics with minor mafic and granitic intrusives. The conspicuous Garibpet Hill is formed of garnet-kyanite-muscovite schist and is surrounded by biotite schist and gneiss.


These faceted garnet beads were collected by local farmers from the Arikamedu site. The samples constitute some of those studied for this report and measure ~4.55-5.5 mm in diameter. Photo by K. Schmetzer.

What about Electron Microprobe Analysis?

During electron microprobe analysis, a sample is bombarded with a beam of electrons. The interaction of the electron beam with the sample material results in formation of X-rays, which can be analysed by the microprobe. The wavelengths and energies of these X-rays provide information about the chemical elements present in the sample (qualitative analysis). When compared with reference materials, the measured x-ray intensities can be used to determine element concentrations (quantitative analysis).

What about a Ternary Diagram?

A ternary diagram is a triangle, with each of the three axes representing a composition, such as the one in this study: pyrope, almandine and spessartine + grossular. The proportions of the three compositions sum to 100%. The plot graphically depicts the ratios of the three variables in as positions in an equilateral triangle. It is used in physical chemistry, petrology, mineral and other physical sciences to show the compositions of systems composed of three species.

The great majority of analysed samples from Arikamedu (beads and fragments) and the rough stones from Garibpet proved to be garnets with a high almadine content. The compositional fields were in close proximity and overlapped to a large extent. Microprobe data revealed almandine in the range of 77-84 mol% with minor components of pyrope, spessartine and grossular. A ternary plot of the molecular percentages of the garnet end members pyrope and almandine and the sum of spessartine + grossular showed that the studied garnets plotted within a relatively small compositional range. This outcome was even clearer when only a small portion of the full ternary diagram was drawn with an extended scale.

(a). This ternary diagram shows the chemical composition of garnets from Arikamedy and Garibpet calculated for the molecular end-members pyrope, almandine and spessartine + gossular. The compositions plot in a concentrated area, except for two anomalous Arikamedu samples (blue and purple arrows) that fall outside the main compositional field, which are inferred to be from different sources. (b). An enlarged detail of the main compositional field for the Arikamedu and Garibpet garnets corresponds to the area defined by the grey triangle in the inset. Note the extensive overlap in the composition of garnets from Arikamedu and Garibpet.

The compositional ranges for the two localities were:

  • Arikamedu: 77.4-83.5% almandine, 10.2-14.2% pyrope, 0.9-5.3% spessartine, 0.9-2.5% grossular
  • Garibpet: 79.2-84.0% almandine, 9.6-12.0% pyrope, 1.1-5.9% spessartine, 0.6-2.1% grossular

How does LA-ICP-MS work?

The LA-ICP-MS analysis process can be thought of in two main parts: material sampling i.e. Laser Ablation (LA) and chemical analysis i.e. Inductively Coupled Plasma Mass Spectrometry (ICP-MS). A tiny, nearly invisible ablation put is caused by the laser, into the girdle of the gemstone. There will be minimal damage as the laser vaporises only a microscopic amount of the sample for analysis. It nebulizes the material and the aerosol produced is transferred in a gas stream to an ICP-MS for elemental and/or isotopic analysis. An ICP-MS combines a high-temperature Inductively Coupled Plasma (ICP) with a Mass Spectrometer (MS). The ICP is an ionisation source where the energy is supplied by electric currents, which ionises the atoms. These ions are then separated based on their mass-to-charge ration (m/Q) and detected by the MS.

What is LA-ICP-MS used for?

LA-ICP-MS is recognised as one of the most important spectrometric techniques and has been used in gemmology for quantitative chemical analysis. It provides data that can be used to create chemical fingerprint diagrams for geographical origin determination.

The compositional averages and the ranges demonstrated by LA-ICP-MS analyses were nearly identical for the Arikamedu and Garibpet garnets. A similar relationship was noted for lanthanide rare-earth elements. Several samples from Arikamedu and Garibpet also showed chemical zoning for some trace elements, such as Y, P and Zn. Considering in detail both trace and other elements, chemical zoning between core and rim was strong for Mn and significant for Ca, largely consistent with the results of microprobe analyses.

How can we classify solid inclusions?

Solid inclusions are divided into three categories, by time of entrapment: those formed before the host crystal, called protogenetic; solids which arise from the solution from which both they and the host originated, called syngenetic and those formed after the host crystal has finished its growth, epigenetic.

The proto - to syngenetic inclusions in the cores comprised, with decreasing abundance: apatite, quartz, ilmenite, rutile, monazite, zircon, graphite and fluid inclusions. At the core-rim boundary, a very characteristic layer of fibrous sillimanite bundles was observed. Isolated zircon, monazite and quartz crystals were also found occasionally in the rims. The garnets were often cut by brownish-yellowish fractures coated by various generations of goethite or other iron oxides-hydroxides.


Coarse acicular sillimanite needles were observed in a small number of the garnets from Arikamedu. Photomicrograph by H.A.Gilg.

The small differences observed in the average chemical compositions between the Arikamedu and Garibpet material can probably be explained by the fact that the Garibpet samples were collected from one secondary source within a large garnet-bearing area and, therefore, are not entirely representative of the Garibpet rough material used for bead production at Arikamedu.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled 'The Linkage Between Garnets Found in India at the Arikamedu Archaeological Site and Their Source at the Garibpet Deposit’ by Karl Schmetzer, H. Albert Gilg, Ulrich Schüssler, Jayshree Panjikar, Thomas Calligaro and Patrick Périn 2017/Volume 35/ No. 7 pp. 598-627

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

Cover image: Faceted garnet bicones from Arikamedu were cut in half and polished for microprobe analysis. Two drill holes meet approximately in the centre of each sample. Photo by H.A.Gilg.

The Fascinating History of Antique Turquoise Jewellery

The Fascinating History of Antique Turquoise Jewellery

In his third Gemstone Conversations column for Gems&Jewellery, Jewellery Historian and Valuer John Benjamin FGA DGA FIRV explores the fascinating history of turquoise and its use in jewellery design from the Shahs of Persia to the Art Deco design movement.

Read more

Birthstone Guide: Garnet For Those Born In January

Birthstone Guide: Garnet For Those Born In January

If you're lucky enough to be born in January, vibrant garnet is your birthstone. A rainbow jewel of the gem world, garnet displays the greatest variety of colour of any mineral and is very often untreated, making it a rarity in the gem world. 

Read more

Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

Read more

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

Read more

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

Read more

Tanzanite: The Contemporary December Birthstone

Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

Read more

Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

Read more

Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Read more

Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

Read more

Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

Read more

 

Additional Info

Read more...

Glass Simulants of Gems and Enhancement of Natural Gem Materials in the 16th Century

Glass Simulants of Gems and Enhancement of Natural Gem Materials in the 16th Century

Guy Lalous ACAM EG, summarises the state of making glass simulants of gems in the 16th Century, as well as exploring the historical information we have available on the enhancement of natural gem materials.

What about gem treatments?

Many gemstones can be treated to alter their colour and clarity. Today, gemmologists are confronted to a broad spectrum of treatments ranging from the simple to the highly sophisticated as well as the easily detected to the highly elusive. Treatments include: bleaching, coating, dying, fracture filling, heating, impregnation, high pressure, high temperature, irradiation, laser drilling and lattice diffusion.

What about the origins of gem treatments?

Heated carnelian was found in Tutankhamun's tomb-dating to at least 1300 B.C.C. Plinius Secundus (First Century A.D) is the earliest written source on gem treatments. Pliny discusses many gemstone-enhancement techniques including foils, oiling and dying that are still in use today, almost 2,000 years later. The "Stockholm Papyrus" made about 400 A.D. in Greek-speaking Egypt contains 73 recipes which deal with the falsification of pearls and gemstones; representing the oldest extended recipe collection dealing with gems. In 1502 "The Mirror of Stones" was published, a fascinating book by Camillus Leonardus, a physician and astrologer of Pesaro, Italy. It discusses gem treatments and simulants and also how to identify those stones that are "not true" and the importance of experience and knowledge in this subject.

In the 17th Century, we have the Gemmarum et Lapidum Historia of 1609 by Boetius de Boot, a physician of Bruges. In the French translation of 1644, there is discussion in Chapters 20 to 22 of the decolourising by heat of sapphire, topaz, amethyst and the like, to produce diamond imitations; the dyeing of stones, mostly with metal compounds, an extended discussion of metal foils and an obscure description on how to harden gemstones. By 1820, agate dyeing in Idar-Oberstein had been perfected to the point that it was practiced on a large scale and the agate sold as treated stone. For the first time, a gemstone material was altered commercially and marketed as such and not as a natural material. By the middle of the 19th Century, gemmology had turned into a science (K. Nassau).

What about man-made glass?

Man-made glass dates back to approximately 5000-4000 BC, this took the form of glazes used for coating stone beads. It was not until 1500 BC that the first hollow glass container was made by covering a sand core with a layer of molten glass. Glass blowing became the most common way to make glass containers from the First Century BC. As from the First Century AD colourless glass was produced and coloured by the addition of colouring agents. Glass has been used as a substitute for emerald and other fine gemstones since at least the days of ancient Rome. Skills for glass making spread throughout Europe and the Middle East when the Roman Empire disintegrated. It was not until the full development of the Renaissance, in the mid-1500s, that a writer purposely gave the tedious details of the entire process of glass 'gem' making. This author was Giovan Battista Della Porta.


Figure 1: These 'emerald' and 'amethyst' glass eardrops in the Renaissance style were presumably assembled in the second half of the 16th century. The mounting is partially silvered copper. The green glass 'gems' are 6 mm in diameter. Courtesy of a private collection near Rome, Italy; photo by Carlotta Cardana.

Giovan Battista Della Porta was the first to publish in print recipes for making glass simulants of gems, in addition to information on the enhancement of natural gem materials. His Magiae Naturalis (1558), originally written in Latin, enjoyed vernacular translations in several European languages. The second, vastly improved edition (Della Porta, 1589), again in Latin, did not enjoy the same popularity - possibly because the first one has saturated the market or, alternatively, because the Catholic Church has enforced rules that made alchemy a forbidden practice and even the title Magiae became suspect. In spite of such restrictions, both editions contributed to making glass 'gems' popular decorative objects and to increasing their trade. During Baroque times, interest in glass 'gem' making reached an acme, and Della Porta's treatise was even translated into English in 1658.

Figure 2: This portrait of Giovan Battista Della Porta at the age of 50 is from the title plate of the 1589 edition of his Magiae Naturalis.

His modus operandi was well known. For every secret he learned, he first checked for other possible sources by reading books by old masters, after which he tested the results by performing experiments in his home laboratory. Glassmaking was one process that could be performed with a kiln, a rather simple apparatus. The preparation of certain special glasses (e.g. coloured ones suitable for simulating gems) involved knowledge that had been an artisan secret until it was released by Della Porta in his original 1558 edition of Magiae Naturalis.

In Book III of Magiae Naturalis, 1558, Della Porta wrote three chapters related to glass that followed the descriptions of other chemical operations, such as sublimation, distillation, purification and melting, plus miscellaneous recipes on how to repair broken corals, pearls and gemstones. He did not care to deal with how to make ordinary glass, but he proceeded directly to release the technicalities on how to prepare the special colourless glass that would be suitable for making coloured glass, so as to imitate gem materials. In chapter 16 he summarised the preliminaries, recommending the use of very finely ground silica mixed with fluxes.

In chapter 17 he made a digression aimed at explaining how natural gem materials acquire their colours and shifted to recipes on enhancing colour by using various natural pigments, by slowly diffusing them from the surface to the bulk of the gem under the slow action of fire. Then he returned to recipes intended to add weight to glass without modifying its hardness. In particular, he recommended adding lead to the already prepared colourless glass only while it melts, so as to increase its brilliance and weight. After another digression, he ended Chapter 18 with a series of explanations on how to obtain attractive 'gem' glass by carefully mixing colourless glass with pigments while it melts. The resulting gem simulants would resemble diamond, emerald, sapphire, pyrope, topaz, olivine, chalcedony, etc. The final recommendation was that the crucible containing the molten mix should be kept under close supervision, as excess heating would make the colour fade away.

In 1589, Della Porta, by now a mature scientist, reworked his Magiae Naturalis, expanding it from four books to 20. The in-folio sized text dealing with gems grew from five ordinary pages to a complete Book VI encompassing 10 dense pages and distributed over 13 chapters. Actually, only Chapters 1 to 5 concern glass gem simulants and Chapters 7 to 13 mostly concern the enhancement of gem materials. Everything is described in much greater detail than in the previous edition. After an introduction, Della Portas begins Chapter 1 with a careful description for the preparation of reagents for glass gem making, beginning with two fluxes. Chapter 2 recalls that silica is the main constituent of any glass gem. The raw silica can be either crystal or flint, or even round pebbles; the best of are said to be those gathered from the river Thames.

Chapter 3 describes in detail the furnace and the instruments to be used and Chapter 4 teaches how to prepare pigments. Chapter 5 is the core of the entire process. Indeed, it is titles "How gems are coloured". The pigments are blended with the previously prepared colourless glass while it is molten, so that they mix homogeneously. The recipe for glass used to simulate emerald is given last because the preparation requires a long exposure to fire. The following chapters describe various enhancements of natural gem materials and then move onto enamels, coloured metal sheets for reflection, etc. The author ends book VI, Chapter 13, with the short but factual statement: "This is all what we experimented on gems so far".


Figure 3: The title page of Magiae Naturalis Libri XX, 1589 edition, shows the titles of all 20 books composing this volume.

The second half of the 17th Century in England was characterised by an economic revival with increasing interest for science in general, including those books penned by 'writers of secrets'. The practice of publishing 'secrets', although unwelcome to many, contributed to the development of both science and the economy. In particular, it is significant that Della Porta's Magiae Naturalis, intended for completely different purposes and contributing only poorly to the 'scientific revolution' because of its still rather alchemical bent, eventually helped speed up the English industrial revolution.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled 'Counterfeiting Gems in the 16th Century: Giovan Battista Della Porta on Glass 'Gem' Making'' by Annibale Mottana 2017/Volume 35/ No. 7 pp. 652-666

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

Cover image: Title page of the first english translation of Natural Magick in 1658, the title page of Magiae Naturalis Libri XX, 1589 edition and the frontispiece of the english translation of Natural Magick, 1658.

The Fascinating History of Antique Turquoise Jewellery

The Fascinating History of Antique Turquoise Jewellery

In his third Gemstone Conversations column for Gems&Jewellery, Jewellery Historian and Valuer John Benjamin FGA DGA FIRV explores the fascinating history of turquoise and its use in jewellery design from the Shahs of Persia to the Art Deco design movement.

Read more

Birthstone Guide: Garnet For Those Born In January

Birthstone Guide: Garnet For Those Born In January

If you're lucky enough to be born in January, vibrant garnet is your birthstone. A rainbow jewel of the gem world, garnet displays the greatest variety of colour of any mineral and is very often untreated, making it a rarity in the gem world. 

Read more

Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

Read more

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

Read more

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

Read more

Tanzanite: The Contemporary December Birthstone

Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

Read more

Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

Read more

Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Read more

Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

Read more

Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

Read more

 

Additional Info

Read more...

Coloured Gemstones from Brazil: Past, Present and Future

Coloured Gemstones from Brazil: Past, Present and Future

Guy Lalous ACAM EG presents us a historical review, and an updated overview, of the Brazilian coloured stone industry. His latest Journal Digest from Gem-A’s Journal of Gemmology (Winter 2017. 35.8) also examines the effects of China’s emergence as a consumer market on Brazil’s gem industry.

Recent information on Brazilian gem occurrences is presented from data compiled by the Brazilian National Department of Mineral Production (DNPM) and other resources.

This suite of morganite (16.69–22.22 ct) was cut from Brazilian rough
Figure 1: This suite of morganite (16.69–22.22 ct) was cut from Brazilian rough. The gems were fashioned into various custom cuts, including (from upper left to lower right) Concave, Super Trillion, Deep Concave and StarBrite. Courtesy of John Dyer; composite photo by Ozzie Campos.

The history of Brazilian gemstones began with the early colonisation period. No gem deposits were found until 1573, when emeralds were discovered in the present-day Governador Valadares area. In the late 17th century, the legendary gold mines of Sabarabussu were discovered, located to the east of the present city of Belo Horizonte.

The ensuing rush then expanded north, unearthing the first diamond deposits a few decades later near the town that would become Diamantina. The exploration for gold and diamonds overshadowed other gem mining activities until the 19th century, apart from the Imperial topaz deposits found nearby in the old Minas Gerais capital of Ouro Preto. It was during this time that the term ‘garimpeiro’, which refers to independent miners, emerged.

Germans originating from Idar-Oberstein immigrated to the present-day state of Rio Grande do Sul, and to the north-eastern part of Minas Gerais, after Brazil’s independence from Portugal in 1822. During this period, Idar-Oberstein was experiencing economic troubles as the production from its agate mines dwindled.

The immigrants brought their cutting and polishing techniques with them, and initiated the first stage of development for the modern Brazilian coloured stone industry.

Among the most notable early finds was the 110.5 kg Papamel aquamarine, discovered in 1910 in the Marambaia Valley, as well as numerous emerald deposits. On the eve of World War II, gem-related activities in Brazil were mainly limited to mining, while the end of the war saw the widespread closure of industrial mineral mines in Minas Gerais. Many of these mines turned exclusively to gem production and, accordingly, local lapidary and other trade activities grew dramatically. The latter half of the 20th century was also marked by the emergence of a Brazilian jewellery industry.

What about Brazilian Emeralds?

Emerald is the green variety of the mineral beryl, coloured by chromium and vanadium.  These trace elements are normally concentrated in quite different parts of the Earth’s crust, but complicated geological processes enable these contrasting elements to find each other, and crystallise into one of the world’s most beautiful gemstones.

Geological Processes of Brazilian Emeralds

Brazilian emerald mineralization belongs to the classic biotite-schist deposit, which was formed by the reaction of pegmatitic veins within ultrabasic rocks. The granitic pegmatite injection makes the emerald crystallise at the contact zone between these chemically contrasted rocks.  The pegmatite brings beryllium, while the ultrabasic rock contains chromium and vanadium, and the reaction between them is called metasomatism.  This reaction is, however, only possible when geological fluids are present enough to ensure the transportation of the elements.  Pegmatite-free emerald deposits, linked to ductile shear zones, are also known in Brazil (G. Giuliani).

Weighing 6.11 cm, this emerald is unusually large for clean material from Brazil
Figure 2: Weighing 6.11 ct, this emerald is unusually large for clean material from Brazil. Courtesy of Paul Wild; photo by Jordan Wilkins.

Brazilian emerald production occurs in three states: Goiás, Bahia, Minas Gerais.  The first emerald site in modern times was discovered in 1912 in the deep south of Bahia State, near Brumado. Other emerald discoveries followed in the early 20th century at Ferros, in Minas Gerais; at Itaberaí, in  Goiás and at Anagé in Bahia.

More recent discoveries include: Itabira (1977) and Nova Era (1988) in Minas Gerais; at Santa Terez­inha de Goiás (1981) in Goiás; and in the Serra de Carnaíba region (1983) of Bahia.  Since the 2000s, Brazil has been ranked third among the world’s leading emerald producers, behind Colombia and Zambia.

Paraiba Tourmaline from Brazil

Paraíba tourmalines represented the first recorded instance of copper occurring as a colouring agent in this mineral. The blue-to green colours are primarily due to copper (Cu2+).


Figure 3: The bright coloration shown by Paraíba tourmaline (here, 5.73 ct) as well as its rarity have contributed to its high value. Photo and stone courtesy of Paul Wild.

Paraíba tourmaline—for its singularity and its profitability— became perhaps the most important discovery of the 20th century. Uncovered for the first time in 1987, in the São José da Batalha district of Paraíba, this tourmaline’s unique ‘neon’ blue-to-green coloration contributed to making it one of the most valuable coloured stones on the global market. Production of Paraíba tourmaline has, unfortunately, dwindled in recent years.

Global Gemstone Trade - The Role of Brazil 

In the late 1990s, Brazilian gems were mainly exported to the United States, Europe and Japan, with gem exports to Hong Kong and India increasing too. Finally, another major player emerged: China. The increase in exports of rough gem material to Asia has become very significant and, for a time, most of it was cut and polished in India and China.

Chinese brokers began to arrive in the quartz-producing areas at the beginning of the 2000s. These brokers bought most of the cheaper gems, contributing heavily to a rise in mining activities. Over the years, Chinese dealers acquired a greater variety of stones, showing a preference for tourmaline, especially rubellite.

One consequence of the increasing trade with China, though, has been the collapse of Brazil’s domestic gemstone cutting and polishing industry for low-value materials. The negative effect on local economies is particularly apparent in north-eastern Minas Gerais. In Teófilo Otoni, of the 2,700 lapidary businesses operating in 1993 only 360 remained in 2005. In 2011, China was the main partner in Brazil’s gem commerce, with 2013 seeing Brazil export 60% by weight, or 25% by value, of its output to China (up to 50% by value if Hong Kong is included).

Brazil, however, still remains significant for cutting higher-value gemstones. Teófilo Otoni and Governador Valadares remain the major gem cutting and trading centres. Various urban centres host the country’s main gem fairs, which are small venues compared with other international shows.

An important factor to note is the informal trading of gems; several hundred individuals travel around the country and acquire rough material to sell later in the metropolitan areas. The widespread adoption of modern technology, such as the internet and digital cameras, has revolutionised traditional methods of trading over the past decade, making middlemen somewhat obsolete.

Other factors have also diminished Brazilian gem production, such as the evolution of the international market. Many African countries now produce a larger variety and quantity of gems than they did a couple of decades ago, and those stones are usually sold at lower prices than those from Brazil. The high cost of mining is another reason for the recent decline in mining and production. According to the DNPM, there were 2,294 gem occurrences in 401 different municipalities as of 2013, with 49.3% of them in Minas Gerais and 19% in Rio Grande do Sul.

What about Santa Maria Aquamarine?

One of the rarest and most expensive varieties of aquamarine, the "Santa Maria", has a deeply saturated blue colour, with no hint of green or yellow. Named in honour of Santa Maria de Itabira where these stones were first discovered, the original deposit is now almost depleted, and today most of the Santa Maria colours are also found in several sub-Saharan countries of Africa, including Mozambique.

The colouring process is due to the Fe2+ - Fe3+ charge transfer. This feature is associated with intense absorption and strong pleochroism, and this process cannot be induced by heat treatment. The stones are nearly colourless in the direction of the optic axis, and intense dark blue perpendicular to the optic axis.

Figure 4: This 22.93 ct aquamarine is from Santa Maria in Minas Gerais. Courtesy of Paul Wild; photo by Jordan Wilkins.

Quartz and Topaz from Brazil 

Quartzes are abundant in Brazil. Stunning crystal clusters, appreciated by collectors, come from hydrothermal veins found in the Curvelo and Corinto areas of Minas Gerais. Rose quartz is found in pegmatites in the north-eastern part of Minas Gerais, while rutilated quartz has been produced mainly in the Novo Horizonte District of Bahia State.

Huge amethyst geodes, and most of the agates of the country, have been mined in abundance in Rio Grande do Sul. Citrine is mined there as well, but most of the citrine produced and exported from Brazil is actually heat-treated amethyst.

Figure 5: This large colour-zoned tourmaline from Brazil weighs 144.75 ct. Courtesy of Paul Wild; photo by Jordan Wilkins.

The country accounts for much of the world’s topaz production. Many stones are colourless, or tinged very light blue, and laboratory irradiation creates the bright blue colour. A post irradiation annealing is performed by a heat treatment of the topaz for several hours at around 200°C. The production of the rare and famous orange to orange-pink Imperial topaz has, however, greatly diminished, and good-quality material is particularly scarce on the local market.

Figure 6: The colour of this Swiss Blue topaz (13.73 ct, StarBrite cut) was produced by laboratory irradiation, using colourless or pale blue starting material from Brazil. Courtesy of John Dyer; photo by Ozzie Campos.

More Gemstones of Brazil 

Beryl production is dominated by light to medium-dark blue aquamarine, of which Brazil may be one of the largest exporters, with other beryls including heliodor and morganite. Most production of chrysoberyl is still located in Minas Gerais and Bahiam, but good quality Alexandrite is now harder to find locally and only small quantities are available. Collectively, Emeralds and other beryls, tourmaline, topaz and all types of quartz are the most widely mined coloured stone resources nationwide.

What's Next for the Brazilian Gemstone Industry?

Improved living conditions in rural areas have led to an evolution of the workforce, which is becoming less interested in, and less reliant upon, mining via illicit operations. Brazilian gem exports remain robust and gem mining is seeing a shift toward bigger and more professional companies.

The quantity of mining areas remains substantial and, probably, to a large extent underexplored. Yet, the future of the Brazilian gem industry may depend more on social issues, global market trends and an ability to establish efficient trade relationships.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled '‘Coloured Stone Mining and Trade in Brazil: A Brief History and Current Status’ by Aurélien Reys 2017/Volume 35/ No. 8 pp. 708-726

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

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Characterising Mexican Amber From the Yi Kwan Tsang Collection

Characterising Mexican Amber From the Yi Kwan Tsang Collection

Guy Lalous ACAM EG is on-hand to summarise some of the more in-depth articles from Gem-A's The Journal of Gemmology. Here, he delves into a feature on Mexican amber and the use of FTIR spectroscopy to determine provenance from the Winter 2017 issue. 

Amber is an organic gem. Organic gems are the products of living, or once-living, organisms and biological processes. Amber derives from fossilised resins produced by prehistoric trees.

Such resins are produced by plants in response to certain circumstances, such as defence against insect pests or protection of wounds.The most important resin-producing plant families are classified among the gymnosperms (conifers) and the angiosperms (flowering plants).

The fossilisation process of amber involves a progressive oxidation, where the original organic compounds gains oxygen, and polymerisation, which is an additional reaction where two or more molecules join together. This process produces oxygenated hydrocarbons, which are organic compounds made of oxygen, carbon and hydrogen atoms.  A peculiarity of amber is that it may perfectly preserve an organism in its original life position. 

What about the Yi Kwan Tsang Collection?

This collection consists of 115 amber samples from Chiapas, many of which contain abundant plant and animal fossil inclusions. The ambers were acquired over a 10-year period (~2004–2013) in San Cristóbal de las Casas, Mexico. The collection was displayed in 2015 at the Tucson Gem and Mineral Show and in 2016 at the Beijing International Jewellery Fair. 

FIGURE 1
Figure 1: This necklace from the Yi Kwan Tsang Collection contains amber beads from the San Cristóbal de las Casas area of Chiapas, Mexico, and was made by Francesca Montanelli (Lutezia Jewels, Stradella, Italy). Photo by Francesca Montanelli.

This article characterises Mexican amber from the Yi Kwan Tsang Collection using a variety of methods, including those not very common in gemmology, such as taxonomy studies and mass spectrometry, using techniques optimised for organic molecules. Some of the data was compared to those obtained from amber samples from the Baltic Sea and the Dominican Republic. 

The most important amber mines in Mexico are located in the area of Simojovel, but there are several deposits elsewhere in Chiapas State. They date back to the Late Oligocene/Early Miocene (23-13 Ma).

The area is characterised by three stratigraphic units that contain amber. From bottom to top, these are the La Quinta Formation (28–20 Ma), the Mazantic Shale (23–14 Ma) and the Balumtum Sandstone (16–12 Ma). Most of the amber deposits are associated with lignites, friable shales and deltaic clays in the sandstone. 

There are hundreds of amber mines in the tropical forest around Simojovel, and the amber is mined manually using hammers and chisels. 

The twenty-seven samples examined for inclusions were transparent and typically ranged from golden yellow, orange and orange-red to dark orange; a few pieces were dark brown, and some displayed a little natural green colouration. Internal features consisted of inclusions, as well as less common colour variations and small surface fractures. The fractures are probably related to stress associated with the polymerisation of the resin. 

All of the pieces were inert to short-wave UV radiation but displayed weak to strong fluorescence to long-wave UV. The RI values were constant (1.540), and they were similar to those of Baltic and Dominican amber. Average SG values were found to be homogeneous and relatively low (1.03). 

What about taxonomic classification, taxa and phylum?

Taxonomic classification is a hierarchical system used for classifying organisms to the species level. A taxa is a group in a biological classification, in which related organisms are classified. Phylum is a taxonomic ranking that comes third in the hierarchy of classification, after domain and kingdom.  Organisms in a phylum share a set of characteristics that distinguishes them from organisms in another phylum.   

What about arthropods?

The largest phylum of creatures on Earth without a doubt is Arthropoda, both in terms of number of species and in total number of individuals. There are nearly 1 million species of Arthropods, with over 90% of them being insects. 

The determination of the taxa of the botanical and animal inclusions was difficult because the species that lived in Chiapas during the Oligocene-Miocene were different from the modern ones.

The most important plant inclusions were represented by a petal and a leaflet of the genus Hymenaea, more precisely the species Hymenaea mexicana, which is now extinct. Animal inclusions were more common. They consisted of arthropods such as winged termites and a planthopper. Isolated termite wings were detected as well.

The presence of isolated termite wings is extremely rare and the planthopper species, Nogodina chiapaneca, has only been found in amber samples from Chiapas. It is an extinct species dated to the Middle Miocene, and it lived in a tropical or subtropical climate. The presence of an arthropod of the genus Ochlerotatus (female mosquito) also indicates an aquatic environment.  

Figure 8aFigure 2: (TOP) A flower petal inclusion of the species Hymenaea mexicana (Fabaceae family, Late Oligocene–Early Miocene; Poinar and Brown, 2002 and Calvillo-Canadell et al., 2010) is seen in this Mexican amber sample. The petal measures 1.1 cm long and 0.7 cm wide, and shows a narrow midrib base and basal laminar lobes with a central vein and branches of secondary veins. It appears completely glabrous (smooth). (MAIN ARTICLE IMAGE) The H. mexicana leaflet in this Mexican amber measures 3.3 cm long and 1.1 cm wide. The surface is glabrous and its veins are not visible in this view. Photomicrographs by V. L. Villani. 

FIGURE 11aFigure 3: A planthopper of the species Nogodina chiapaneca (order: Hemiptera, family: Nogodinidae; Solórzano Kraemer and Petrulevicius, 2007) is shown at the bottom of this sample. It measures 11 mm long, and displays a rounded head and a clearly visible thorax with one foreleg. The wings have several veins, but the scales are not preserved. This species is known only from Chiapas amber. Photomicrograph by V. L. Villani. 

FIGURE 12Figure 4: These winged termites and isolated wings (rare in Mexican amber) of the order Isoptera (reclassified as part of Blattodea) were likely trapped at the beginning of the wet season, when termites start to swarm and then shed their wings. The length of the wings is ~1.1 cm. Photo by V. L. Villani. 

What about X-ray powder diffraction?

X-ray powder diffraction (XRD), is an instrumental technique that is used to identify minerals, as well as other crystalline materials.The method is based on the scattering of x-rays by the crystals. X-rays are diffracted by each mineral differently, depending on what atoms make up the crystal lattice and how these atoms are arranged. An X-ray scan provides a unique "fingerprint" of the mineral.

Natural resin/amber is amorphous, so XRD analysis does not yield information on the amber itself but can identify mineral inclusions. XRD identified very small amounts of refikite and hartite, as well as calcite.  Calcite was identified by a diagnostic peak at 3.03 Å (or 29.8° 2θ) in the amber from Chiapas only.  Refikite and hartite have a composition similar to that of resin, but possess a crystalline structure.  They are probably associated with the polymerisation process of the resins. 

What about amber classification?

Amber can be classified according to two criteria: their place of origin, and their chemical composition. When succinic acid is present the amber is classified as succinate, when succinic acid is lacking it is considered as a resinite. 

Mass spectrometry was performed to deter­mine the presence of free succinic acid in the amber samples. Confirmation of succinic acid is obtained from the m/z 117 ion (the negative ion mass peak) corresponding to (M-OH)– of succinic acid. The mass spectrum of the Mexican amber did not show the m/z 117 ion, so the level of succinic acid in this amber was lower than the limit of quantisation (1 ppm by weight), classifying it as a resinite.

The Dominican sample showed a spectrum very similar to that of the Mexican amber, indicating the absence of succinic acid, while the m/z 117 ion was clearly identifiable in the spectrum of the Baltic amber sample.

What about Infra-red spectroscopy and the “Baltic Shoulder” in Baltic amber?

Infra-red spectroscopy is the most effective scientific method for identifying fossil resins.  With this technique, broad absorptions will be witnessed in Baltic amber in the 1260-1160 cm-1 range.  Those are assigned to C-O stretching vibration.  These features known as “Baltic Shoulder” are specific to Baltic amber and are related to the presence of succinic acid. 

Three wavenumber ranges that are important for amber characterisation are 3700–2000 cm–1, 1820–1350 cm–1 and 1250–1045 cm–1; these regions are associated with hydroxyl and carbonyl groups and to C=C double bonds. 

FIGURE 15Figure 15: FTIR spectroscopy of a Mexican (Chiapas) amber shows typical peaks at: 3600–3100 cm–1 (broad absorption band due to the O-H stretching vibration); 2965 and 2860 cm–1 (C-H stretching); 1740 cm–1 (C=O stretching, esters and acid groups); 1450 and 1380 cm–1 (C-H aliphatic hydrocarbons); 1260–1030 cm-1 (C-O stretching aromatic esters and secondary alcohols); and 846 cm–1 (C-C stretching of unsaturated olefins). 

The fossil inclusions observed in Chiapas amber in this study are consistent with a sub-tropical forest and FTIR spectroscopy was confirmed as a useful technique to determine the provenance of the amber samples.   

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Characterization of Mexican Amber from the Yi Kwan Tsang Collection‘ by Vittoria L. Villani, Franca Caucia, Luigi Marinoni, Alberto Leone, Maura Brusoni, Riccardo Groppali, Federica Corana, Elena Ferrari and Cinzia Galli 2017/Volume 35/ No. 8 pp. 752-765 

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

The Fascinating History of Antique Turquoise Jewellery

The Fascinating History of Antique Turquoise Jewellery

In his third Gemstone Conversations column for Gems&Jewellery, Jewellery Historian and Valuer John Benjamin FGA DGA FIRV explores the fascinating history of turquoise and its use in jewellery design from the Shahs of Persia to the Art Deco design movement.

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Birthstone Guide: Garnet For Those Born In January

Birthstone Guide: Garnet For Those Born In January

If you're lucky enough to be born in January, vibrant garnet is your birthstone. A rainbow jewel of the gem world, garnet displays the greatest variety of colour of any mineral and is very often untreated, making it a rarity in the gem world. 

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Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

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Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

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Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

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Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

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Tanzanite: The Contemporary December Birthstone

Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

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Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

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Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

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Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

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Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

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Journal Digest: Radiocarbon Age Dating on Natural Pearls

Journal Digest: Radiocarbon Age Dating on Natural Pearls

Guy Lalous ACAM EG is on-hand to summarise some of the more in-depth articles from Gem-A's The Journal of Gemmology. Here, he explores an article on radiocarbon age dating of natural pearls from the Winter 2017 issue. 

This article describes how radiocarbon age dating can be adapted to the testing of historic pearls. The authors, Michael S. Krzemnicki, Laurent E. Cartier and Irka Hajdas, have developed their sampling method so that radiocarbon age dating can be considered as quasi non-destructive. The refined sam­pling process allows researchers to work with tiny amounts of nacre powder (~2 mg) taken from a drill hole without any damage to the outer surface of a pearl. In this article, pearls originating from a historic shipwreck were submitted to radiocarbon age dating. 

A small selection of pearls (approximately 2–8 mm diameter) from the Cirebon shipwreck was investigated for this study.
The pearls are shown on a historic map of the Java Sea, where the shipwreck was discovered. Photo by Luc Phan, SSEF.

The 10th century Nan-Han shipwreck was discovered accidentally in 2003 off the northern coast of Java, Indone­sia, near the city of Cirebon. The excavation of the ancient merchant vessels produced Yue ceramics, glassware and Chinese coins dating from the 10th century CE, jewellery, loose gemstones and also a number of carved gastropod shells and pearls. The thousands of glass fragments, and several unbroken blue and green glass objects found in the Cirebon shipwreck, undoubtedly originated from the Islamic Middle East. 

This indicates extensive trade in Southeast Asia along maritime routes at that time, which the Cirebon merchant vessel was a part of. This also supports a Persian Gulf origin for the pearls. The partly abraded and brown-to-grey altera­tions around the drill holes of these pearls suggests that they might have been in use for some time, strung on strands or set with metal lin­ings in jewellery before they sank in the vessel with the rest of its cargo. The coins and artefacts provided good evidence for a 10th century age of the shipwreck. 

What about X-radiography?

X-radiography is an imaging technique. X-rays are located beyond UV in the electromagnetic spectrum, where they have even shorter wavelengths and greater energy. Materials of low atomic weight allow x-rays to pass through easily and, therefore, appear dark on x-ray film, and those of high atomic weight block x-rays and appear white.

What about X-ray luminescence Computed Tomography?

X-ray luminescence is an emerging technology in X-ray imaging that provides functional and molecular imaging capability. This emission-type tomographic imaging modality uses external X-rays to stimulate secondary emissions, which are then acquired for tomographic reconstruction. This modality surpasses the limits of sensitivity in current X-ray imaging. 

What about EDXRF?

X-Ray fluorescence analysis using ED-XRF spectrometers is a commonly used technique for the identification and quantification of elements in a substance.

What about X-ray computed microtomography?

Seeing inside a material object, in three dimensions, is often crucial for proper characterisation, so that the link between microstructure and properties can be made. Micro-computed tomography or "micro-CT" is X-ray imaging in 3D on a small scale with massively increased resolution. It really represents 3D microscopy, where a very fine scale internal structure of an object is imaged non-destructively.

Fourteen pearls were analysed routinely by X-radiography and X-ray lumines­cence, as well as by EDXRF spectroscopy. Four of them were selected for X-ray computed microtomography (micro-CT) analysis. 

These 14 pearl samples (71742_A–N; approximately 2–8 mm diameter) from the Cirebon shipwreck were examined for this study.
They are partially abraded around their drill holes and show some brown to grey colour alterations. Photo by Luc Phan, SSEF.

Based on their X-radiographs, trace-element com­position and lack of luminescence to X-rays, the samples studied for this report were all saltwater natural pearls. The separation of natural from cultured pearls is mainly based on the interpretation of their internal structures. The radiography and micro-CT scans revealed that their internal structure main­ly consisted of fine ring structures typical of natu­ral pearls.

What about radiocarbon age dating?

The collision of high-speed neutrons produced by cosmic ra­diation with the nucleus of nitrogen results in the capture of a neutron and the expulsion of a pro­ton, thus transforming the 14N isotope into the radionuclide 14C. The radiocarbon, present only in trace amounts in the atmosphere, combines with atmos­pheric oxygen and forms radioactive carbon di­oxide, which is then incorporated into plants by photosynthesis and subsequently into animals via respiratory and metabolic pathways. As a consequence, the radiogenic 14C is incorporated into the endo- or exoskeletons (e.g. bones or shell structures) of animals.

After death, the lifelong exchange of carbon with the environment suddenly stops, resulting in a slow radioactive decay of 14C in the dead plants and animals. By measuring the ratio of radiogen­ic and stable carbon isotopes (14C/12C), it is thus possible to determine their age. The so-called half-life of 14C (that is, the time at which only half of the original 14C is still present in a sample and, as such, represents the constant rate of de­cay over time) is about 5,700 ± 40 years.

What about MICADAS?

The MICADAS is a mini carbon dating system through accelerator mass spectrometry. It is a two-step process. The first part involves accelerating the ions to extraordinarily high kinetic energies, and the following step involves mass analysis. The system allows radiocarbon analyses of ultra-small samples with great accuracy in only a couple of hours’ time. 

A pearl is a calcium carbonate (CaCO3) con­cretion formed by bio mineralisation processes in a mollusc—very much the same processes as for shell (exoskeleton) formation. As such, pearls (and shells) contain carbon, mainly the stable iso­tope 12C (as well as 13C) but also a small fraction of radiogenic 14C. The carbon used for the bio mineralisation of pearls and shells mainly originates from two very different carbon pools: (1) oceanic dissolved inorganic carbon; and (2) respiratory CO2, mainly stemming from food metabolism.

As such, the so-called marine reser­voir age effect may distinctly affect the resulting 14C ages of shells and pearls, especially in areas with upwelling of ‘old’ water.  Hence, a correction is required to take into account the geograph­ic location of the sample. The 14C/12C ratio was measured on three samples using the Mini Carbon Dating System (MICADAS).  

The calculated 14C age BP was corrected by apply­ing a marine reservoir correction that was based on values for the Java Sea location. These were estimated (mean-weighted) based on 10 data points in the vicinity of the sampling site. The result corresponds approximately to the end of the 10th century, which cor­relates well with the age stipulated for the coins, pottery and other artefacts found in the shipwreck. 

 Age determination can support evidence of historic provenance in the case of antique jewellery and iconic natural pearls. It can also be used to identify fraud in cases where, for example, younger pearls are mounted in historical jewellery items, or have been treated so that they appear older rather than having been farmed during the 20th century. 14C age dating can be used to obtain evidence to support a decision whether a pearl is of natural or cultured formation. This is because methods to commercially cultivate pearls from certain mollusc species only began during the 20th century.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Radiocarbon Age Dating of 1,000-Year-Old Pearls from the Cirebon Shipwreck (Java, Indonesia) by Michael S. Krzemnicki, Laurent E. Cartier and Irka Hajdas 2017/Volume 35/ No. 8 pp. 728-736

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

The Fascinating History of Antique Turquoise Jewellery

The Fascinating History of Antique Turquoise Jewellery

In his third Gemstone Conversations column for Gems&Jewellery, Jewellery Historian and Valuer John Benjamin FGA DGA FIRV explores the fascinating history of turquoise and its use in jewellery design from the Shahs of Persia to the Art Deco design movement.

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Birthstone Guide: Garnet For Those Born In January

Birthstone Guide: Garnet For Those Born In January

If you're lucky enough to be born in January, vibrant garnet is your birthstone. A rainbow jewel of the gem world, garnet displays the greatest variety of colour of any mineral and is very often untreated, making it a rarity in the gem world. 

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Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

Read more

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

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Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

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Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

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Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

Read more

Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Read more

Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

Read more

Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

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Additional Info

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Journal Digest: Delve into the Colours of Rainbow Lattice Sunstone

Journal Digest: Delve into the Colours of Rainbow Lattice Sunstone

Guy Lalous ACAM EG returns with his latest Journal Digest, which summarise a scientific article from Gem-A's Journal of Gemmology. Here, he explores an article on rainbow lattice sunstone from Australia featured in the Spring 2018 issue, Volume 36 No. 1. 

The feldspars are a family of silicate minerals which occur in igneous rocks. They contain variable amounts of Na, K and Ca and are divided in two solid solution series: plagioclase (albite – anorthite) and alkali feldspar (orthoclase – albite). Both orthoclase and plagioclase boast of a sunstone feldspar variety, however, the name ‘sunstone’ refers to the gem’s appearance rather than to its chemical makeup.

What is aventurescence and adularescence?

Aventurescence, is a ‘sparkly, metallic-looking luster caused by flat, reflective inclusions’ (GIA). Adularescence is the term applied to gems exhibiting a sheen or schiller effect caused by the intergrowth of two different feldspars, such as moonstone. The colours seen in such material depend on the thickness of the layers involved, with the thicker ones giving rise to colours from the red end of the spectrum and the thinner ones colours from the blue end.

Figure 1: Rainbow lattice sunstone displays conspicuous colourful patterns that are produced by light reflecting at a specific angle from inclusions.
The gold ring on the left contains a 6.17 ct sunstone and the polished fragment on the right weighs 15.00 ct. Courtesy of Rainbow Lattice; photo by Jeff Scovil.

A rare gem feldspar known as rainbow lattice sunstone exhibits both aventurescence and adularescence, with the added presence of oriented elongate and triangular mineral platelets. The authors (Jia Liu, Andy H. Shen, Zhiqing Zhang, Chengsi Wang and Tian Shao) examined this sunstone using microscopy, electron microprobe and XRD analysis, magnetic measurements, SEM-EDS and Raman spectroscopy.

 Figure 2: The six samples of rainbow lattice sunstone obtained for this study display a combination of optical effects
including adularescence (e.g. sample 1), aventurescence (see samples 1 and 6, in particular) and a rainbow lattice effect, which can be seen using different lighting conditions and directions.
The samples weigh 0.29–3.57 g. Composite photos by J. Liu.

Rainbow lattice sunstone is found in the Harts Range, north-east from Alice Springs, Northern Territory, Australia. The Harts Range comprises a complex assemblage of granite gneiss, marble, calc-silicate, amphibolite, psammite and pelite that have been metamorphosed to upper amphibolite to granulite facies (Huston et al., 2006). The metamorphosed sedimentary rocks are intruded mainly by granite, granodiorite and metamorphosed mafic rocks of uncertain origin.

The igneous rocks are generally associated with widespread metasomatic granitisation. The granodiorite in the Bruna gneiss unit contains pegmatites in which K-feldspar occurs, and the mining area is crosscut by quartz veins and pegmatite outcrops.


Figure 3: The adventurescence phenomenon in the sunstone is produced by pseudohexagonal reddish brown platelets of hematite.
A) exhibits sparkling appearance in reflected light. B) Images taken from sample 1. Photomicrographs by J. Liu.

The typical aventurescence of sunstone can be seen with magnification. In their analysis, Liu et al., found that the scattered reddish-brown platelets showed pseudohexagonal and rhomb-shaped morphology. The lattice-forming inclusions in the sunstone consisted of orangey brown to black elongate and triangular plates. 

The orangey brown ones displayed colourful reflections when viewed with oblique pinpoint lighting. Some of the samples displayed obvious adularescence when observed using various incident light angles. Gemmological testing of the rainbow lattice sunstone revealed average RIs of 1.518 to 1.540, and an SG of 2.58, which are consistent with those of orthoclase.

Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS)

As noted by Katrin Field Inc., ‘viewing three-dimensional images of microscopic structures only solves half the problem when analysing samples. It is often necessary to collect more than imaging data to be able to identify the different elements in a specimen.’

SEM images of the black inclusions of magnetite in rainbow lattice feldspar.
Yellow line designates the remaining areas of magnetite, while the dashed red lines show the inferred outline of the original platelets. Images by J. Liu.

Utilising the SEM-EDS Microscope system, we can examine micro-scale and nano-scale features with magnification up to 300,000x and detect chemical composition. 

The black platy, elongate or triangular inclusions consisted of thin sheets with nanometer-scale thickness. Qualitative SEM-EDS analysis revealed that these inclusions contained Fe and O, but no Ti was detected in the present study.

A tiny fragment of the sunstone containing only one black inclusion that was strongly attracted to a Nd-Fe-B magnet, which proved that the black inclusion consisted of an oxide of iron that possessed strong magnetism.  The presence of magnetite as the ferromagnetic material is consistent with the SEM-EDS analysis. 

Electron Microprobe Analysis

Electron microprobe analysis is an analytical technique that is used to establish the composition of small areas of a specimen. As Dr. Nilanjan Chatterjee of MIT’s Electron Microprobe Facility notes, this method is non-destructive and utilises X-rays excited by an electron beam incident on a flat surface of the sample. 

This article found that the quantitative chemical compositions and end-member components of the host feldspar, analysed with the electron microprobe, showed 96% orthoclase and 4% albite. 

The black to orangey brown, platy, elongate and triangular inclusions that produce the rainbow lattice effect are shown in both transmitted (a, b) and reflected (c–f) light. Photomicrographs by J. Liu.

X-ray diffraction (XRD)

XRD is an instrumental technique that is used to identify minerals, as well as other crystalline materials. The method is based on the scattering of x-rays by the crystals. In a 2014 article published on XRD Analysis: Principle, Instrument and Applications, M.S. Pandian observed how: ‘X-rays are diffracted by each mineral differently, depending on what atoms make up the crystal lattice and how these atoms are arranged....’ An X-ray scan ‘provides a unique "fingerprint" of the mineral’. In the sunstone analysed by Lui et al., the XRD pattern of the host feldspar identified it as orthoclase (Or96Ab4).

Raman spectroscopy

Raman Spectroscopy is a testing technique using infrared, visible or ultraviolet light to identify different specimens. When this monochromatic light is applied at a specific frequency via a laser, the Raman spectrometer detects the re-emission of photons.

Most of the photons pass through the material, but some interact with the molecules and modify the vibration of the atomic bonds. Most photons are scattered with no change to their energy, but a small number of photons lose energy to the molecules, and even fewer gain energy. This energy difference (Raman effect) is a distinctive feature of particular molecules, and can be used to identify them.

As such, the Raman effect is different for each material, based on its molecular structure and we can use individual emission spectrums to identify different gemstones. This is because a number of energy values are associated with gem species, and these are expressed by waves per cm and are viewed as a series of peaks. These peak patterns form a database for us to identify not only gemstones, but to identify inclusions and treatments.


 This representative Raman spectrum of the host feldspar.

Raman spectrum of the host feldspar showed peaks at 455, 474 and 513 cm–1 which is proof of its identity as orthoclase. The main Raman shifts of the reddish-brown platelets causing the aventurescence and the orangey brown lattice-forming inclusions were 226, 245, 297, 411, 500, 612 and 1319 cm–1, which match with hematite. The main Raman shifts of the black lattice-forming inclusions were 303, 538 and 664 cm–1, which fit nicely with magnetite.

Conclusions 

Sunstone inclusions may be composed of hematite, ilmenite, magnetite, native copper or goethite. The appearance of the aventurescence phenomenon depends on the size of the inclusions. Small particles produce a reddish or golden sheen, while larger inclusions create an attractive, glittery appearance.

The lattice appearance displayed by rainbow lattice sunstone is created by inclusions of hematite and magnetite. These minerals form very thin blades that occur within planes of a single orientation at different levels in the feldspar (like pages in a book).

Platelets of hematite also produce aventurescence. Viewed with reflected light, the aventurescence is illuminated from one direction, while the colourful lattice effect appears when the lighting is shifted to a different angle.

The magnetite and hematite predominantly form triangular shapes or elongate blades with terminations that are parallel to the triangular directions. The magnetite inclusions in many cases have oxidized to hematite, corresponding to the iridescence or rainbow effect across the lattice patterning. In contrast, the unaltered magnetite is black with a metallic sheen.

Unfortunately, there is not much of this material available in the market.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Revisiting Rainbow Lattice Sunstone from the Harts Range, Australia’ by Jia Liu, Andy H. Shen, Zhiqing Zhang, Chengsi Wang and Tian Shao 2018/Volume 36/ No. 1 pp. 44-52

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

The Fascinating History of Antique Turquoise Jewellery

The Fascinating History of Antique Turquoise Jewellery

In his third Gemstone Conversations column for Gems&Jewellery, Jewellery Historian and Valuer John Benjamin FGA DGA FIRV explores the fascinating history of turquoise and its use in jewellery design from the Shahs of Persia to the Art Deco design movement.

Read more

Birthstone Guide: Garnet For Those Born In January

Birthstone Guide: Garnet For Those Born In January

If you're lucky enough to be born in January, vibrant garnet is your birthstone. A rainbow jewel of the gem world, garnet displays the greatest variety of colour of any mineral and is very often untreated, making it a rarity in the gem world. 

Read more

Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

Read more

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

Read more

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

Read more

Tanzanite: The Contemporary December Birthstone

Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

Read more

Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

Read more

Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Read more

Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

Read more

Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

Read more

 

Additional Info

Read more...

Journal Digest: Bumble Bee Stone from Indonesia

Journal Digest: Bumble Bee Stone from Indonesia

Getting to grips with The Journal of Gemmology Volume 36, No.3, Guy Lalous ACAM EG offers us an edited breakdown of a feature on Bumble Bee Stone from West Java, Indonesia. 

Bumble Bee Stone (BBS) is an attractive bright yellow-to-orange and black patterned gem from West Java, Indonesia. Production has been ongoing since 2003, yielding an estimated 150 tons of lapidary material. 

The paper in The Journal of Gemmology reports on the location, geology and gemmological properties of BBS, focusing on the cause of its extraordinary colouration. 

The mining area for BBS is situated on the lower slopes of an active volcano, Mt Ciremai (or Cereme), which is located about 25 km south-west of the coastal town of Cirebon in West Java.

What is a solfatara?

A solfatara is a volcanic area producing hot vapour and sulfurous gases.  The name is derived from the Solfatara of Pozzuoli a volcano part of the Campi Flegrei (Burning Fields) near Naples. 

BBS was formed within a solfatara (a fumarole that vents gases rich in sulphur) occurring in close proximity to the Mt Ciremai volcano.

This type of vent is common near active stratovolcanoes, and results from the heating of circulating groundwaters containing various elements or compounds extracted from the volcanic system—in this case iron, sulphur, calcium carbonate and arsenic.

Figure 2. The BBS deposit is located at the base of Mt. Ciremai, an active volcano, in the vicinity of Kuningan, West Java, Indonesia. 

Such a system produces abundant ‘sooty’ pyrite, consisting of very small crystals of the iron sulphide, crystallising rapidly near the surface, which looks like black soot.

As the gases escape the solfatara, minerals are deposited as more-or-less regular bands in fractures within the volcanic rock, which is here comprised of fine-bedded volcanic ash and intercalated pyroclastic tuff (an accumulation of volcanic ejecta of varied size).

The veins are near vertical, with individual colour bands rarely exceeding 5 cm. The tuffaceous wall rock contains marcasite, an iron sulphide that is unstable when exposed to air and moisture.

After about 1–2 weeks in this environment, the breakdown of marcasite facilitates the separation of BBS vein material from its volcanic host rock.

 
Figure 3. The pulverulent appearance of the colour banding in BBS is shown here under magnification in sample no. 1591.
Left: Yellow-to-orange pigment is disseminated within the carbonate matrix, forming colour bands, within with micro-geodes are found. 
Right: The black pigment forms minute discs that are resolvable only at higher magnification. Photomicrographs by E. Fritsch. 

The gemmological properties of the material are reported. Refractive index values were difficult to measure because of the material’s porosity.

Specific gravity varied between 2.42 and 2.74; the average of seven measurements was 2.57. Bubbles appeared when a drop of diluted hydrochloric acid was placed on the surface, confirming that this material is carbonate rich.

Therefore, it is not jasper, which is an opaque form of microcrystalline quartz. When examined with the binocular microscope, the appearance was that of coloured powders cemented in the carbonate matrix.

Small cavities were actually micro-geodes; many contained very small crystals that were orange or tended towards red. Visible-range reflectance spectra for the various coloured areas of BBS revealed that the yellow regions have an absorption edge at ~530 nm, the colour perceived is a combination of all wavelengths above 530 nm (green), which is observed as yellow. 

The absorption edge shifted towards 550 nm in the orange areas which is logical as less yellow equates to more red. The black portions showed a nearly flat spectrum.

What about pararealgar?

Pararealgar is a bright yellow monoclinic polymorph of realgar, a more common and better-known red arsenic sulphide, which is also monoclinic but with a different class of symmetry.

The Raman spectra of all analysed samples presented bands for calcite at about 285 and 160 cm–1. Aragonite bands were found in only one part of one orange sample. In ‘mustard’-to-yellow parts, additional major bands were recorded at about 346, 284, 233 and 157 cm–1.

They correspond to pararealgar, which has the formula As4S4. The Raman spectrum of realgar dominated the bright orange areas and the reddish crystals in the micro-geodes mentioned above, with main peaks at approximately 355, 221, 194 and 184 cm–1.

It had a much higher Raman scattering intensity than pararealgar, thus it dominated the spectrum of a mixture of both polymorphs.

As such, the orange areas appeared to be a mixture of pararealgar and realgar. In the black areas, a small Raman signal appeared at about 378 cm–1. The closest signal we could find is pyrite, FeS2, which has a strong doublet at 385 and 355 cm–1.

 

Figure 5. The Raman spectra of the BBS samples tested always showed peaks for calcite, with additional features corresponding to pararealgar and realgar in yellow and orange areas. The spectrum shown here for realgar is from a reference crystal, as no area of BBS we analysed consisted of pure realgar.

What about framboidal pyrite?

The term framboid describes an aggregation of uniformly-sized particles of the same mineral, from the French framboise for raspberry, as this berry is composed of aggregated uniformly sized drupelets.


Figure 6. The three polished specimens of BBS on the left (12–26 cm long) show spectacular orbicular (also called ‘bull’s-eye’) patterns with a strong colour contrast. The typical bull’s-eye pattern ranges from 10 to 14 mm in diameter, but may be much larger (see Figure DD-1 in The Journal’s online Data Depository). Such pieces are derived by cutting across botryoidal areas such as shown by the BBS slab on the right. The colourless calcite crystals (up to ~3 cm long) induce the botryoidal structure in the overlying sulphide-bearing layers. Photos by J. Ivey.

Scanning Electron Microscopy and Microanalysis were performed on the material.

The nearly ubiquitous presence of Ca confirmed that the material is mostly calcite, although the consistent presence of a small Mg peak suggested it was a slightly magnesian calcite.

Yellow-to-orange areas were rich in S and As. The black discs were composed of circular aggregates with maximum diameters ranging from ~5 to 30 μm.

They were composed of micrometer-sized crystals identified as pyrite. Their morphology varied, from nearly cubic (square faces) to cubo-octahedral (pseudo-hexagonal faces) to octahedral triangular faces. This is reminiscent of framboidal pyrite.

What about botryoidal concretions?

A concretion is a compact mass of a mineral formed by precipitation within the spaces between sediment layers. Botryoidal refers to the crystal shape resembling a bunch of grapes. 

BBS belongs to a rare category of gems coloured by micro-inclusions of sulphide pigments.

Its most remarkable characteristic is a bright yellow colour, which is caused by the presence of an unexpected sulfide, pararealgar. The orange colour in some samples consists of a mixture of pararealgar and realgar. Both minerals are polymorphs of As4S4, arsenic sulphide.

This is not the first time that pararealgar has been identified as the colouring agent in a gem material. Gaillou (2006) documented this pigment via Raman scattering in a little-known bright yellow opal from the area of Saint Nectaire in central France.  

BBS is currently a single-source gem material, with no other deposit having been documented elsewhere.

The most sought after is an orbicular variety with a typical bull’s-eye pattern, obtained by slicing across botryoidal concretions.

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Bumble Bee Stone: A Bright Yellow-to-Orange and Black Patterned Gem from West Java, Indonesia’ Emmanuel Fritsch and Joel Ivey 2018/Volume 36/ No. 3 pp. 228-238

Cover Image: These cabochons (up to 40 mm long) and rough pieces of Bumble Bee Stone (BBS) were produced in 2017, the samples are more orange than previously mined material.  
Photo by J. Ivey.

Interested in finding out more about gemmology? Sign-up to one of Gem-A's courses or workshops.

If you would like to subscribe to Gems&Jewellery and The Journal of Gemmology please visit Membership.

The Fascinating History of Antique Turquoise Jewellery

The Fascinating History of Antique Turquoise Jewellery

In his third Gemstone Conversations column for Gems&Jewellery, Jewellery Historian and Valuer John Benjamin FGA DGA FIRV explores the fascinating history of turquoise and its use in jewellery design from the Shahs of Persia to the Art Deco design movement.

Read more

Birthstone Guide: Garnet For Those Born In January

Birthstone Guide: Garnet For Those Born In January

If you're lucky enough to be born in January, vibrant garnet is your birthstone. A rainbow jewel of the gem world, garnet displays the greatest variety of colour of any mineral and is very often untreated, making it a rarity in the gem world. 

Read more

Getting Started with Quartz Inclusions

Getting Started with Quartz Inclusions

Do you know your calcite inclusions from your dumortierite, epidote, fluorite and rutile? Here, Charles Bexfield FGA DGA EG explores some incredible quartz inclusions and explains what to look for when shopping for quartz specimens.

Read more

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

Iridescence has to be one of the most mesmerising and magical optical effects seen in gemstones. But have you ever wondered how it occurs? Gem-A's Collection Curator Barbara Kolator FGA DGA shines a light on this fascinating optical effect and tells us about the gems that are most likely to display it.

Read more

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Gem-A Gemmology Tutor Pat Daly FGA DGA offers us a glimpse at some of the more unusual items in Gem-A's Gemstones and Minerals Collection.

Read more

Tanzanite: The Contemporary December Birthstone

Tanzanite: The Contemporary December Birthstone

Are you looking for the perfect festive gift for a December baby? Gem-A tutor Lily Faber FGA DGA EG considers tanzanite – one of three birthstones for December – and shares how this relatively new gemstone compares to its purple and blue-hued rivals.

Read more

Birthstone Guide: Turquoise For Those Born In December

Birthstone Guide: Turquoise For Those Born In December

Beautiful blue turquoise is one of three birthstones for the month of December (in addition to zircon and tanzanite). It is enriched with real cultural significance that can be traced back thousands of years. Here, we explore the blue shades of turquoise and explain what makes this gemstone so special...

Read more

Understanding the Cat's Eye Effect in Gemstones

Understanding the Cat's Eye Effect in Gemstones

Chatoyancy is the gemmological name given to the curious optical effect in which a band of light is reflected in cabochon-cut gemstones, creating an appearance similar to light bouncing off a cat's eye. Gem-A's Collection Curator, Barbara Kolator FGA DGA explains chatoyancy and highlights some of the many gems in which it can occur.

Read more

Jade and its Importance in China

Jade and its Importance in China

Jade has long been revered by gem lovers internationally, but nowhere more so than in China. But what is it that makes this gemstone so special? Gem-A's Assistant Gemmology Tutor Dr Juliette Hibou FGA gives us an overview of jade, how to identify it and its significance in Chinese culture.

Read more

Highlights of Gem-A Conference 2019

Highlights of Gem-A Conference 2019

The Gem-A Conference is always the highlight of our gemmological calendar! If you didn’t manage to make it, we’ve put together a few of the highlights from this year’s event to fill you in on what you missed, and whet your appetite for Gem-A Conference 2020!

Read more

 

Additional Info

Read more...
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