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.


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


 

Subscribe to our Newsletter
Keep updated with Gem-A
  • Gemmology Events
  • The Journal of Gemmology
  • Gem-A News&Blogs
Sign Up

Get in Touch
Can't find what you're looking for?
  • The Gem-A Headquarters are conveniently located next to Hatton Garden
  • Opening Hours: Monday to Friday 9:00-17:00
Contact Us