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Journal Digest: Radiocarbon Age Dating on Natural Pearls

Published in Gem Knowledge

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 10^thcentury 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 10^th 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

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

The Fascinating History of Antique Turquoise Jewellery

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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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Jade and its Importance in China

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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: Delve into the Colours of Rainbow Lattice Sunstone

Published in Around the World

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^–1which 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

Birthstone Guide: Garnet For Those Born In January

Getting Started with Quartz Inclusions

Understanding Iridescence: Opals, Pearls, Moonstones and Fractured Stones

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.

Tanzanite: The Contemporary December Birthstone

Birthstone Guide: Turquoise For Those Born In December

Understanding the Cat's Eye Effect in Gemstones

Jade and its Importance in China

Highlights of Gem-A Conference 2019

Additional Info

Journal Digest: Bumble Bee Stone from Indonesia

Published in Around the World

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.