First published in the Spring 2018 issue of Gems&Jewellery, Harold Killingback FGA explores chatoyancy in sillimanite cabochons, an optical phenomenon where a band of light, known as a 'cat's eye', appears to hover above the surface of a stone, resulting in a striking lustre and colour.

Figure 1 shows a sharp cat’s eye in a sillimanite cabochon lit by a single point source (the sun) above the stone, i.e. from the same side from which it is viewed.

There is also a sharp ‘eye’ when the light comes up through the stone, toward the viewer, as shown in Figure 2. One can even see the ‘eye’ when the cabochon, lit from below, is viewed horizontally along the line of the stone’s major axis, see Figure 3.

 Figure 1. Epi-chatoyancy, plan view, lit by the sun.
In addition to the ‘eye’ line, there are reflections of the sun and of surroundings from the top surface of the cabochon.
All Image Credits: Harold Killingback FGA.

For these views, the single point of light was a fibre optic ‘pipe’, with an added tube 157 mm long and 10 mm in internal diameter, to fit the pipe. This acts as a collimator – a device that narrows a beam of particles or waves (including light).

Figure 2. Dia-chatoyancy, plan view.
Light from a fibre optic source below enters the stone through a suitably shaped hole in a black paper mask.
 As well as the “eye” line, the fibrous nature of the stone is visible.

It is less usual to see the effects in Figures 2 and 3 (dia-chatoyancy) than that in Figure 1 (epi-chatoyancy), partly because we normally admire gems when lit from above rather than from below, so we may simply miss seeing the effect.

Partly, also, because many chatoyant stones are too opaque to let much light pass right through them. Epichatoyancy, by contrast, can be the result of reflection from parallel inclusions immediately below the surface, and so the light path within the stone can be shorter, resulting in less absorption.

It helps, in this case, that the base of this cabochon is fairly smooth, rather than being left rough ground as is often the case, so more light from below can enter without being scattered.

Figure 3. Dia-chatoyancy, end view. The cabochon is resting on a glass slide so that fibre optic light can enter from below.
The background is illuminated by LED light, hence its blueness. Here, also, the fibrous structure of the stone can be seen, orientated at right angles to the bright line.

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Sillimanite, with andalusite and kyanite, is a polymorph of aluminium silicate. Its RI is in the range 1.654 -1.683 and DR 0.020 (1). A fibrous form, sometimes called Fibrolite, can exhibit chatoyancy due to its own structure. This example shows such a form.

The chatoyancy of the brown sillimanite from Orissa, India, is said to be due to ultrafine long and short ilmenite needles (2). The stone illustrated might well be from this source as the dealer said it was from India, but he could not be more precise.

I have not, however, been able to see any needles in this example. I put it near a strong magnet, but no attraction was detected. This stone measures 8.5 x 6.1 x 3.7 mm and weighs 1.75 ct.

Ray Diagram

The problem in drawing a ray diagram for chatoyancy is that one does not know where the relevant reflective surface is located within the stone, nor what angle it is orientated.

The solution is to work backwards along the exit ray, and forwards along the incoming ray, and then to see what has to happen where their paths cross if they are to be parts of a single ray.

The resultant diagram is shown in Figure 4, which represents a partial cross-section of the stone along its major axis as viewed from the side. In this example, I have chosen to examine the case of the dia-chatoyancy when the stone is viewed horizontally along the line of its major axis, as in Figure 3.

Arbitrarily, I have selected the horizontal ray leaving the cabochon at the point where the normal to the surface rises at 30º to the horizontal.

   
Figure 4. Ray diagram. Arbitrarily, an exit ray has been chosen, and the associated ray before refraction has been calculated.
Another arbitrary choice is shown for the entry ray.  The condition that these rays be one continuous ray is met by a reflective surface as shown.

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As the exit ray is horizontal, the angle of refraction, r, must be 30º. The incident angle, i, was calculated using Snell’s law, and an RI of 1.658. The resultant angle is 17.6º, and so the ray in the stone slopes up from horizontal at an angle of (r – i), which equals 12.4º in the case illustrated.

The arbitrarily chosen entrant ray is not refracted, as it is normal to the base. The angle between it and the exit ray just examined is 90 + (r – i). For these rays to be parts of a single path there must be a reflective surface where they meet. Let it be tilted at θ to the horizontal. As the incident ray at the reflecting surface is vertical, the angle of incidence is also θ, and the angle of reflection must be the same.

So 2θ = 90 + (r – i). In our example we get θ = 51.2º. This angle would be the same had I chosen any other ray in the vertical entrant beam.

We can repeat the calculation of θ corresponding to various exit points of horizontal rays from the stone as defined by the angle of the normal to the surface at these points. The results are shown below.

Figure 5 is included for fun, really, but it does illustrate the fact that multiple point sources produce multiple eye-lines. Here, there are but two sources and so two eyes. In the limit, a cloudy sky consists of infinite point sources and produces, not any lines at all, but so many that no individual line can be distinguished and the stone reflects an even glow.

Figure 5. Bi-dia chatoyancy. The white line is produced by a fibre optic incandescent light from below the cabochon.
The blue line is from a LED torch, also from below, but slightly to one side of the other source.
The camera was set to correct for the red cast of tungsten light, so the LED ray looks very blue by comparison. Note the fibrous nature of the stone.

Conclusions

I have calculated the angles of reflector surfaces for only one direction of view and only one of sillimanite’s two values of RI. These reflector angles could all be provided if the reflecting inclusions were roundish in cross-section, whether hollow tubes or fibres.

If, however, the inclusions are crystalline and polygonal in cross-section, one is forced to the conclusion that the orientation of the needles about their long axis must be random, in contrast to the fact that this axis must, in all cases, be orientated in the direction dictated by the structure of the material.

An alternative explanation is given by Moon and Phillips (3). They ascribe the effect as being due to Fraunhofer diffraction, applying Babinet’s Principle of Complementary Apertures. The needles can then be regarded as apertures of the same size. The crosssection of the needles is then of no consequence.

This paper has also demonstrated that where, as here, it is not possible to trace a ray from start to finish, one can trace a known emergent ray backwards as far as possible, and then trace a known entry ray forward. If these rays cross, one can deduce what could unite these rays into a single path.

References

1. Identification of Gemstones, pp 254-255. O’Donoghue and Joyner.

2. Photoatlas of Inclusions in Gemstones, Vol. 3, p 626. Gübellin, E. and Koivula, J. I.

3. A. R. Moon and M. R. Phillips, Schweiz. mineral. petrogr. Mitt. 64, 329-334, 1984.

This article originally appeared in Gems&Jewellery Spring 2018/ Volume 27/ No.1 

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: Bi-dia chatoyancy in cabochon. Image Credit Harold Killingback FGA. 

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Understanding the Cat's Eye Effect in Gemstones

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From the Spring 2018 issue of Gems&Jewellery, Rui Galopim de Carvalho FGA DGA explores how ‘alternative facts’ have resulted in an informal nomenclature that permeates the world of gemmology. Here, he offers some examples of these long-standing quirks in terminology.

In gemmology there are a number of trade names and expressions that lack accuracy. However, written or verbal tradition has given them a wide visibility and many have become part of our trade’s informal nomenclature.

Here are a few examples. This article does not intend to set standards or correct any existing article or gemmological text. This review has resulted from the careful reading of multiple books and journals in the course of normal study processes and peer review editing to verify or validate information.

More than pointing out apparent mistakes or erroneous information, this approach has shown there is a balance between purist or scientific lexicon and the more trade-orientated nomenclature, which is characteristic of gemmology that serves an industry and its consumers.

READ MORE: The 'Cat's Eye' Phenomenon - Exploring Chatoyancy 

It also demonstrates that there are terms that emerge from colloquial tradition and not necessarily from academia. For this article, a selected number of situations have been chosen to illustrate the misuse of expressions and names, although in some cases, tradition wins over picky nomenclature.

These are ‘alternative facts’ in the sense that, for the more conservative and orthodox reader, they can be considered ‘fake news’.

FLAME FUSION ‘SYNTHETIC’ SPINEL

One of the most popular artificial products that gemmologists learn how to identify in their education is the flame fusion, or Verneuil, synthetic spinel.

These have been around for almost a century and were produced in many colours (except red) to imitate, not natural spinel, but rather other gem materials such as diamond, aquamarine or blue sapphire to name a few (O’Donoghue, 2005). 

The gem and jewellery community, and almost all literature, simply describes these products as ‘synthetic spinel’ (O’Donoghue, 2006, Matlins, 2003, Liddicoat, 1993, Hodgkinson, 2015).

A ca. 1960 Portuguese brooch in 19.2k gold with a flame fusion blue ‘synthetic spinel’. According to trade regulations, one could not call the material a ‘synthetic’ but rather an artificial stone.
Photo: Carlos Pombo Monteiro © Arquidiocese de Évora/ Fundação Eugénio de Almeida.

The trade, however, has a very strict definition of ‘synthetic’ and CIBJO – The World Jewellery Confederation – defines synthetic stones as “artificial products having essentially the same chemical composition, physical properties and structure as that of their naturally occurring counterparts” (CIBJO Gemstone Book, 2016).

It happens that every gemmologist knows that the gemmological properties of natural spinel differ slightly from the properties of the flame fusion counterpart, due to the fact that they do not have the same chemical composition, with a different alumina to magnesia ratio (Al2O3 / MgO), that is 1/1 in natural spinel and usually 3.6/1 in the flame fusion product (Rinaudo, 1997).

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In strict observation of the CIBJO rules, we could not call these products ‘synthetic spinels’ as they do not meet the criteria for a synthetic stone.

This is, however, a typical case when a verbal or written tradition overtakes the formality of a nomenclature rule. Despite the noncompliance with the trade standards, it is widely accepted and tolerated that these products can be named, tagged and traded as ‘synthetic spinel’.

MARCASITE AS A GEM MATERIAL

In the gemmological world we refer to marcasite as an opaque, golden material with a metallic lustre, often cut in very small and simple rose-type pieces. Its first record goes back to the 18th century (Pouget, 1762) with its heyday as a gem material in the 1930s (Bartlett, 1997). The name marcasite drives from the Arabic or Moorish word used to describe pyrite and other minerals (Anthony, 1990).

As an actual fact, the name shouldn’t be marcasite, but pyrite as this material is the well known cubic iron sulphide (FeS2). Today, every major text book acknowledges the fact that the material is pyrite and that marcasite is an accepted trade name in the jewellery community (Webster, 1994, Hodgkinson, 2015).

Marcasite is in fact the IMA — International Mineralogical Association’s approved name for a distinct mineral that has no use in jewellery whatsoever, occasionally causing confusion to a more scientifically-educated public.

A ca. 1940 silver pin set with a paste and small round shaped marcasites, known as pyrite in the scientific community.
Photo: Carlos Pombo Monteiro © Arquidiocese de Évora/Fundação Eugénio de Almeida.

Marcasite and pyrite are two distinct materials in spite of being polymorphs of iron sulphide, with marcasite crystallising in the orthorhombic system and pyrite in the cubic system (Klein, 1993).

These polymorphs were defined as separate mineral species in 1845 when marcasite was proposed as a new species (Heidiger, 1845) but some authors and museums reportedly had difficulty in separating them (Bannister, 1932), a task that was greatly solved by Bragg in 1914 with the introduction of X-ray diffraction in mineral identification.

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In the jewellery world, the word marcasite kept being used in the traditional way despite the acknowledgment of its true mineral classification as pyrite, which makes this another typical case of an erroneous name kept due to trade tradition (Bartlett, 1997).

‘ORGANIC’ GEMS?

Since the dawn of gemmological education in the second quarter of the 20th century, gem materials have been commonly organised into separate categories, like diamonds, coloured gemstones and organics.

Within organics we see a list of gem materials that originate from the activity of living organisms, e.g. ivory, bone, coral, tortoiseshell, pearl, mother-of-pearl, shell, horn, corozo (vegetable ivory) and copal just to name the most important ones (Pedersen, 2004).

Pearls, like these natural colour bead-nucleated cultured pearls from Fiji, are examples of gem materials that should be named biogenic and not organic due to their composition.
© J Hunter Pearls Fiji.

It happens though that the word ‘organic’ has specific meanings, rather than simply being the broader statement of material generated by a living organism (e.g. a carbon based compound).

Moreover, some of the gem materials grouped as organics are not, in any sense, organic in composition and that is the case of precious corals, pearls, cultured pearls, mother-of-pearl and shell.

The major composition of the materiais is bio-mineralised calcium carbonate in aragonitic and/or calcitic structures. Carbonates, as crystal matter, are strictly speaking considered inorganic matter, not organic (Strack, 2006).

At the 2016 CIBJO Congress, these arguments were discussed and there was a consensus that a better word to describe gem materials that originate from living organisms would be ‘biogenic’, literally meaning that they result from biological activity. 

A full list of references is available upon request. All Image Credits: Rui Galopim de Carvalho.

This article originally appeared in Gems&Jewellery Spring 2018/ Volume 27/ No.1 

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: A ca. 1960 Portuguese brooch in 19.2k gold with a flame fusion blue ‘synthetic spinel’. According to trade regulations, one could not call the material a ‘synthetic’ but rather an artificial stone. Photo: Carlos Pombo Monteiro © Arquidiocese de Évora/ Fundação Eugénio de Almeida.

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Most of us know about fluorescence in gemstones, but how many use it as part of their gemmological testing routine? Here, Gem-A gemmology tutor Lily Faber, FGA DGA EG, delves deeper into fluorescence and explains why it can be both enlightening and enjoyable for gemmologists.

Luminescence in Gemmology

When we use the term luminescence in gemmology, it generally refers to the term photoluminescence, which is the emission of a cold, visible light when a gem material (or general substance) is excited by light of a shorter wavelength. Two examples are fluorescence and phosphorescence.

Fluorescence occurs when a gem material is illuminated by radiation of shorter wavelengths with higher energy.

A bag of cubic zirconia under LWUV with areas of blue fluorescence that highlights the presence of diamonds. 

The visible light emitted stops when the source of illumination is turned off. Phosphorescence, on the other hand, is a visible light that is emitted by a gem material after the original source of exciting radiation has been switched off.

The Hope Diamond Phosphorescence

A famous example of a gemstone that strongly phosphoresces is the blue Hope Diamond, which glows a bright red for several minutes after being excited by short wave UV light. Both fluorescence and phosphorescence can have varying strengths from very strong to weak. If a material does not either fluoresce or phosphoresce, it is considered inert.

Quartz under LWUV showing oil inclusions.

The History of Gemstone Fluorescence

Fluorescence has been observed for years, but it was not until Sir George Stokes extensively documented this effect in relation to gemmology that it officially became part of the scientific lexicon. In 1852, Sir George coined the word fluorescence, named after fluorspar, more commonly known as fluorite, which is a highly fluorescent material.

The ‘Stokes Law of Fluorescence’ or ‘Stokes Shift’ states that the fluorescent emission of light will always be that of a longer wavelength than the excitation source, i.e. the light emitted is of a lower energy than its excitation source.

Why Use Fluorescence to Test Gemstones?

Fluorescence can be a helpful tool when used correctly. Some gemstones have a characteristic or, very rarely, a diagnostic reaction to UV light. One gemstone that notably both fluoresces and phosphoresces is a diamond, which typically fluoresces blue in longwave UV light and then phosphoresces yellow.

This is a diagnostic result for a colourless to yellow diamond in the Cape series (Type Ia), but please be aware that fluorescence is rarely diagnostic as reactions may vary wildly within the same species or variety of gemstone.

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Fluorescence can indicate or confirm the identity of a stone. For example, citrine quartz is inert to fluorescence due to the presence of iron, which eliminates fluorescence. If you are testing a yellow stone that may potentially be a citrine, and it fluoresces orangey-yellow under LWUV and red under SWUV, it cannot be a citrine and is more likely to be a scapolite.

Scapolite from Ontario under LWUV.

Other reasons to use fluorescence? It is quick and normally takes less than one minute to observe reactions. You can test gemstones that are loose, set, rough or fashioned, and you can test either single gemstones or multiple gems at the same time. Finally, it is entertaining!

How Does Fluorescence Occur in Gemstones?

Ultraviolet light (UV) is the most commonly used excitation source. We cannot see UV light as it sits just below the visible light spectrum (400nm- 700nm) at 10-400nm. UV light enables us to see fluorescence because a gem material will absorb this radiation source and then emit light that is lower in energy and therefore visible to the eye. But what is actually happening within the gemstone itself to elicit such a colourful reaction?

It has to do with electrons. When electrons are excited by a source of radiation, they jump to a higher energy level around the nucleus of the atom. The excited electron remains in this excited state for a short period of time until it falls back to its original ground state. As the electron returns to its ground state, it emits energy either as heat or as visible light (fluorescence).

A: Natural spinel, red paste, synthetic verneuil ruby, almandine garnet and two natural rubies.
B: The same stones under LWUV. C: The results under SWUV. 

If you are wondering if all minerals fluoresce, the answer is no. Only 15% of all known mineral species exhibit this effect, the causes of which can be very complex. One of the more better-known and documented causes is the presence of activator elements that can be excited by higher energy wavelengths.

Some activators include chromium (Cr), uranium (U), manganese (Mn), lead (Pb), titanium (Ti) and rare earth elements (REE). Some elements are considered to be the complete opposite, however, and when present they eliminate or quench fluorescence, causing a gemstone to be inert. Common quenchers include iron (Fe) and nickel (Ni).

 
Kunzite under SWUV.

Using UV Light to Test Gemstones

The types of UV light used in testing are long wave (LWUV) with a principle wavelength of 365nm, and short wave (SWUV) with a principle wavelength of 254nm. Different testing equipment ranges from UV keyrings (typically LWUV) to a UV viewing cabinet. When using UV light to test gemstones, it is important to remember that any exposure to UV light can damage your eyes, but particularly use caution when using SWUV as it is more dangerous than LWUV. Always wear protective UV goggles or ensure that your UV cabinet is installed with an eyepiece that filters out UV light.

To properly use a UV keyring, take the following steps:

• Never look directly into the light
• Turn off surrounding lights so you are in a dark environment.
• Place the gemstone table-down if facetted. If table up, the gemstone may reflect the UV light into your eyes and creating confusing, conflicting or inconclusive results.
• Hold the keyring approximately two inches away from the stone and, if testing multiple gems, always be consistent with the distance at which you hold the light.
• Record whether the stone is inert or fluorescing, and the strength of the reaction.

Sometimes you may see some dull purple or red light in the gemstone or on a few facet edges. This means that the gemstone is reflecting the purple UV light and is not itself a fluorescent reaction.   

A: Synthetic verneuil sapphire, scapolite, natural sapphire, topaz and citrine. B: Under LWUV. C: Under SWUV 

Understanding Fluorescence Results

While fluorescence is not a diagnostic test, and results can vary dramatically even within the same gemstone species (variable emerald results, for example), it can be a useful indication of what a gemstone is. When testing diamonds and colourless, transparent simulants, keep the below chart in mind.

When testing red to pink gemstones,do keep in mind that natural rubies in particular may have variable fluorescence based on their iron content. If iron is present, the ruby will have minimal to no fluorescence. Synthetic rubies tend to have much stronger fluorescent reactions.

When testing green gemstones, fluorescence can be tricky to use for identification purposes. However, it may be useful in terms of recognising the presence of fillers in emeralds or green jadeite jade, for example.

Some resin fillers fluoresce a whitish colour under LWUV and if this reaction is seen in either of the aforementioned stones, it may be an indication that filler is present, especially if the fluorescence is concentrated in seams or certain areas rather than being evenly distributed across the stone.

Note the natural emerald fluorescing a whitish colour (second from right), hinting that a resin filler may have been used. Further testing will be needed to confirm this possibility.

Left to Right: synthetic flux emerald, synthetic hydrothermal emerald, natural emerald, chrome diopside.

Additionally, natural emeralds, if they do fluoresce, will have a red to inert reaction under LWUV, and a weaker inert or green reaction under SWUV. Synthetic emeralds may fluoresce red or, in the case of the synthetic hydrothermal emerald, they may be inert if doped with iron to imitate a natural inert reaction.

Untreated green jadeite does not fluoresce, so any other reaction should be regarded with suspicion and further testing will be needed. When testing yellow gemstones the bottom chart may prove useful.

Conclusion - Fluorescence in Gemstones

As is evident, fluorescence can be a helpful tool when testing gemstones, though not always diagnostic. It is a quick test that is one of the more exciting ones in the world of gemmology. ■

All images courtesy of Lily Faber and Gem-A gemmology tutor, Pat Daly.

This article originally appeared in Gems&Jewellery Summer 2018/ Volume 27/ No.2 

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: Willemite and calcite fluorescing under SWUV.

---

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

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Read more

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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

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Read more

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Read more

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Read more

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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

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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

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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

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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From the Summer 2018 issue of Gems&Jewellery, Gagan Choudhary FGA, deputy director of Gem Testing Laboratory Jaipur, describes the various types of fake gem rough that has passed his desk in recent months, including mica rock presented as emerald rough, cubic zircona and topaz fashioned as diamond octahedtrons, and synthetic quartz mimicking aquamarine.

Reaching directly to the miners for procuring rough has always been profitable, but involves a huge amount of risk unless one has enough experience in buying at the source, deep knowledge about the stone being purchased, and handling the pressure thereof.

Often, there have been cases when dealers tend to forget the possibilities of scams and frauds at mining sites or the markets nearby. The sellers at such locations often present glass, synthetics, treated gems or other cheap natural materials as expensive gems in order to make some quick money. This practice has been prevalent at most of the major mining regions around the world for decades.

At Gem Testing Laboratory Jaipur, we routinely encounter such cases, some of which are presented here:

GLASS-FILLED MICA-ROCK, PRESENTED AS EMERALD ROUGH

Recently, a 1,075 gm black micaceous rock was presented for identification (1), a true example of a fraudulent rough, which, although not shocking to us, was definitely an interesting one. Initial observations with unaided eyes from different angles suggested the presence of several crystals, with hexagonal profile, embedded in the rock.

Such rock formation is a common sight for those dealing in emerald rough, especially from locations where emerald is associated with mica (phlogopite) schist, such as Zambia. Careful observation using strong fibre-optic light surprised us. Under reflected light, only small areas or corners of the embedded crystals appeared green.

The rest appeared dark, due to the presence of black mica on and around crystals.

1: This 1075 gm micaceous rock was embedded with elongated ‘hexagonal’ crystals of artificial glass (marked with arrows).
Note the difference of texture around embedded crystals and rest of the rock.

However, when light was transmitted through these crystals, they appeared bright green, which raised suspicion about their origin. Such bright green colour under transmitted light, especially in an embedded crystal, had never been seen before. Further examination revealed a granular texture around these embedded crystals, while the rest of the rock appeared flaky; this supported our suspicion.

These features suggested that micaceous rock was first drilled, filled with green ‘hexagonal’ crystals artificially, and then the joints were covered with a mixture of glue and black mica.

When observed under ultraviolet light,  corners of the embedded crystals (micafree areas) fluoresced chalky-yellow green. Raman analysis confirmed these embedded crystals as artificial glass. 

MICA-COATED GLASS IMITATING EMERALD ROUGH

Another form of emerald-rough imitation are these mica-coated glass (2). In this case, pieces of green glass are first fashioned in the shape of hexagonal rough, which is then coated with fine powder of black mica mixed in glue, followed by a layer of mica chips. These worked-up pieces are then taken to the mining sites by the middlemen and mixed in parcels of low-quality natural emeralds.

The illustrated glass specimens here were seen in a parcel of emeralds from Jharkhand, India.

 2. Glass samples worked-up to imitate emerald rough by fashioning into hexagonal crystals and coating with black mica.
Found mixed in a  parcel of natural emerald. 

SYNTHETIC RUBY FROM MOZAMBIQUE

We came across a small parcel of rough rubies (five pieces, weight range of 3.60- 18.06 ct) submitted for identification. All the specimens were tumbled with a corroded surface and interestingly coated with a yellow-brown substance. Most of the samples were free from inclusions, but under immersion microscopy all displayed curved growth lines, characteristic of synthetic ruby grown by Verneuil process (3).

Appearance of these specimens clearly suggested that they were presented as natural. Prior to this we have seen many more specimens of synthetic ruby, and in much larger sizes, presented as natural. As per the discussion and information from the depositor, these stones were purchased in Mozambique.

3. Rough Samples weighing 3.60-18.06 ct were identified as synthetic ruby. 
note the presence of yellow-brown substance on the extreme right, imitating mud on natural rough

NATURAL AND SYNTHETIC RUBY COMPOSITE

This 28.73 ct bright red rough, associated with some black and white minerals, was presented as a natural ruby. Upon initial examination with unaided eyes, the surface displayed some areas of milky angular zones against a pinkish to purplish background, typically seen in natural ruby crystals.

When examined under transmitted light, a large central area of the specimen appeared bright red, while the edges appeared dark and opaque (4). This raised suspicion about the origin of this rough.

Careful examination under the microscope revealed a sudden change. of growth and inclusion patterns, not only in the core and surface, but also within the surface; the surface displayed small chips with different inclusion patterns.

In addition, distinct colour variation between the core and edges of the specimen was evident. These features suggested that the specimen is a composite where a transparent piece of synthetic ruby is covered with small chips of natural ruby.

4.This bright purplish red-pink rough(top) is a composite of synthetic and natural ruby. The central part is a synthetic ruby while the out part is composed of chips of natural ruby.
Under transmitted light (bottom) the central synthetic part of the specimen appeared bright red, while the edges appeared dark and opaque.

SYNTHETIC SAPPHIRE, PRESENTED AS NATURAL ROUGH

Synthetic counterpart is a common imitation for natural sapphire rough; these are presented in two forms — one as broken, tumbled rough, and second, as fashioned, well-formed hexagonal-pyramidal crystals with surface markings (cover image). Although, their identification is not challenging in a gem lab, they might pose problems while buying at the mines.

The specimen illustrated in (5) was found mixed in a parcel of sapphires, purchased in Madagascar.

5. These two crystals, weighing 63.93 (left) and 44.66 (right) ct displaying bipyramidal habits and associated white mineral, were submitted as sapphire.
The crystal on left was identified as sapphire, but one on the right as glass.

GLASS AS SAPPHIRE ROUGH

Two blue crystals weighing 63.93 and 44.66 ct, as illustrated in were submitted together. Both crystals displayed bi-pyramidal habits and associated white mineral, typically seen in corundum. Interestingly, there was an obvious difference in colour and transparency of both the crystals; the crystal on the right had much better colour and transparency.

Closer inspection of the bright blue crystal revealed hemispherical cavities on its surface, coloured swirls and numerous gas bubbles — the features associated with glass. The grey-blue crystal (5, left) was proved to be natural sapphire, while crystal habit and associated white mineral (kaolinite) suggested Kashmir as its origin.

CUBIC ZIRCONIA AND TOPAZ AS DIAMOND OCTAHEDRON

Cubic zirconia as diamond imitation, both rough as well as cut, have been in existence for a long time, however, in recent years colourless topaz has become a frequent encounter in diamond imitation, especially in rough form. Image 6 illustrates one such example, where the left specimen is a cubic zirconia while the right one is topaz.

6. Cubic Zirconia (left) topaz (right).

These stones are fashioned as typical crystal forms associated with diamond, here, octahedron; often striations, grooves or triangular markings are created on these fashioned octahedrons, giving them a natural appearing crystal.

In the recent past, this author has encountered some large packets of such created ‘topaz octahedrons’, being presented as diamond.

Separation of cubic zirconia from diamond was easily done on the basis of higher specific gravity, while topaz by its anisotropic optic character. Although identification of these imitations is straightforward, when buying at mines or open markets one has to be careful. 

TREATED QUARTZ AS EMERALD ROUGH

In addition to the glass discussed above, emerald rough is often imitated by coated (7) or dyed quartz.

There have been cases where transparent quartz is painted with green colour and presented as emerald, however, as illustrated in 7 (left), such materials can be separated by crystal form (prism and rhombohedral faces) and horizontal direction of grooves or striations on prism faces.

Another material is the quartzite variety, which is first dyed green, then fashioned as hexagonal crystal shape to imitate emerald; such fashioned crystals are often coated with black mica too.

Even body colour, translucency and absorption spectrum (band at 650 nm) can separate such dyed materials from natural emerald.

 
7. Quartz crystals painted with green colour and coated with black mica are presented as emerald rough.
Also note the horizontal direction of grooves or striations on prism face.

SYNTHETIC QUARTZ AS AQUAMARINE CRYSTAL

This is one of the most unusual materials this author has seen for making a fake crystal — synthetic blue quartz fashioned as an hexagonal crystal of aquamarine (8). The crystal was fashioned into six-sided prisms, with pyramids and basal pinacoids — a crystal form commonly seen in aquamarine.

The crystal also contained a conical-tube with brown epigenetic material (such as iron oxide filling) visible to the unaided eyes. On observing the crystal from different sides, it displayed two parallel planes (seed plate) with colourless area and an attached metal clamp. Such features are often seen in synthetic quartz and other gems grown by hydrothermal process.

When viewed from the top i.e. down the ‘c’ axis, the interfacial angles between the prism faces ruled out the possibility of natural crystal form associated with crystals belonging to hexagonal crystal system, such as beryl.

As per the precision at which the nature operates, opposite sides of prism faces are parallel to each other, while in this case no opposite sides were parallel. Identification of this specimen as synthetic quartz was established on the basis of ‘bull’s eye’ optic figure, seed plate and infra-red spectra.

Such cases remind us of the importance of studying crystallography, not only for the identification of gem rough, but also in the creation of fake rough, which the maker of this fake missed out on.

8. Left: blue specimen presented as aquamarine was a synthetic quartz fashioned into a hexagonal crystal, terminated by pyramidal and pinacoidal faces. Also note the conical tube containing brown epigenetic substance. 
Right: the top view of the synthetic quartz specimen

CONCLUSIONS

Fake rough is an inevitable part of the gem trade, and the scams associated with this are increasing day-by-day. Identification of rough, especially in the field is quite challenging, however, one needs to keep in mind the existence of fake material in local markets or even mines.

Careful inspection of the presented rough before making a buying decision is always advisable, keeping in mind the crystallographic features. ■

All images courtesy of the author.

This article originally appeared in Gems&Jewellery Summer 2018/ Volume 27/ No.2 

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: Synthetics presented as natural rough tumbled sapphire. Image Credit: Gagan Choudhary. 

---

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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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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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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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From the Summer 2018 issue of Gems&Jewellery, Gem-A Associate Member and co-owner of Moriarty's Gem Arts, Steve Moriarty, shares his fascinating career in gemmology.

Although a gem specialist’s career can often start confined to a classroom or a lab, pouring over stone samples and peering through a microscope, our industry also offers wonderful opportunities to travel around the globe.

Steve Moriarty is not only the co-owner of Moriarty’s Gem Art in Crown Point, Indiana, but also an experienced gem-cutter and gem explorer. His family-run business, including the websitesmoregems.com, tanzanitejewelrydesigns.com, opallust.com and mothersfamilyrings.com, was founded in 1975, and today Steve, his wife Nancy, and two of their three children work together to offer a gem-orientated retail experience. Add to this Steve’s 25-years as a professional gem-cutter and it is clear that he has seen, worked with and sold some incredible gemstones.

READ MORE: Journal Digest, Blue Zircon

“Ever since I was young, I collected rocks and fossils,” Moriarty comments. “I really got interested in the gem business while I was in college; my brother Tom started importing gems from India and would send me stones, which I took to the jewellery arts classes to sell.”

Despite a stint as a chemist, Moriarty opted to follow his passion for gems and in 1975 he joined his brother selling coloured gems to jewellers in the Midwest United States. In these early years, Moriarty says that “colour was not important to most jewellers,” which made businesses like his rare in the region. “My wife Nancy and I started on our own as Moriarty Gem Corporation in 1984 and I began travelling overseas to Thailand for cut gems.” By 1994, with his passion for travelling, buying gems and selling them to retailers waning, Moriarty established his own retail store – now Moriarty’s Gem Art – in Crown Point, Indiana.

A rough garnet and its cut and polished counterpart. Image Courtesy of Steve Moriarty and Moriarty's Gem Arts.

He says: “Cutting gems, creating jewellery, custom orders and four websites is almost more than we can handle at times! I love to cut gems, this is my first priority. But to sell enough of those gems you need to do something with them, and many of the gems end up [as] unique shapes, so most everything is a custom piece. The greatest difficulty my designers and I have is we want to be artists and create our vision, but most often we have to recreate the customer’s vision, or what they saw on Pinterest or somewhere else online.”

READ MORE: Heritage Series -Maggie Campbell Pedersen FGA ABIPP

Despite this, the frustrated gem artist in Moriarty is usually restored by incredible finds and far-flung travels. When asked to describe his most inspiring trips, Moriarty points to the early 1980s, when a trip to Kenya revealed that the border to Tanzania had just been opened to American tourists. “The next day we were off on the five hour bus ride from Nairobi to Arusha, and my love of tanzanite began,” he says.

On a later trip in the early 2000s, Moriarty and his long-time travelling colleague, Jim Fiebig, discovered an “amazing cornucopia of gems” in Madagascar. He says: “There was so much amazing material that we were rejecting great gems just because they were priced a little high compared to the abundant bargains we were getting. I had many great trips to Madagascar after that, buying many of the finest quality gems I have ever owned, but never again in such quantity as the first trip.”

Today, Moriarty focuses his freeform carving attentions on Ethiopian opal, which can “require up to two full days to get a decent polish” because of the undulating surfaces.

READ MORE: Investigating Fake Rough

In addition, Rwandan amethyst, carved by Moriarty, recently appeared on the cover of Gem-A’s The Journal of Gemmology (Vol.36 No.1). Although amethyst is not high on Moriarty’s carving ‘wish list’, this material was particularly impressive. “I had been cutting Uruguay material for 20 years and considered it to be amongst the finest in the world, but when I saw the intensity of the secondary red and blues [in the Rwandan amethyst], I was hooked.”

Moriarty also mentions daylight fluorescent hyalite opal from Zacatecas, Mexico — another find that truly inspired him. After discovering some specimens at the Denver Gem & Mineral Show, he returned home and ended up cutting 7.27 carats — one of the largest faceted opals of this type. “We posted our video on YouTube just before we went to Tucson and someone shared it on reddit.com. In three days, it had been viewed over a million times. Currently, total viewing is over three million.”

Left: Hyalite opal from Zacatecas, Mexico, fluorescing in the daylight. Right: the same opal under UV light.

He continues: “I spent some time looking for hyalite opal to compare pricing, but was unsuccessful finding any cut stones in Tucson 2018. One dealer who specialises in rare gems, did give me an idea of the price [as he had] sold one of five carats. This led me to immediately call my office and have them put the hyalite in the safe and take it offline. It seems this material, discovered in 2013, was mined out by 2016 and was very unique for its characteristic of daylight fluorescence. Our next call was to the dealer who sold me the rough and we met and purchased anything of quality that he had left.”

READ MORE: Heritage Series - Let's Bragg About It!

There is an excitement to this lifestyle, of chasing down gems, that is particularly appealing to those getting started in the field. However, Moriarty also enjoys cutting his ‘old favourites’. He explains: “I love cutting garnets and although the prices for garnets of unusual colours have gone up dramatically, I still think they are under-valued. When I get done with a garnet, to me, it looks as good as any diamond I have ever seen.”

Despite his love of travelling, Moriarty is enjoying the fact that his latest obsession – Oregon sunstone – is much closer to home. And who can blame him for wanting a little more time? After all, there are gems to cut, custom orders to fulfil and a wealth of websites to be cared for… it is all in a day’s work for the Moriarty family. ■

All images courtesy of the author.

This article originally appeared in Gems&Jewellery Summer 2018/ Volume 27/ No.2 

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: Cover of Gems&Jewellery Summer 2018. Image Credit: Gem-A. 

---

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

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

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

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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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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

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 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

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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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From the Summer 2018 issue of Gems&Jewellery, Gem-A gemmology tutor Beth West FGA DGA EG explores the Musica people and the emeralds of Colombia. 

There is a brooch displayed at the heart of the Geology, Gems and Minerals Gallery in the Smithsonian Institution in Washington, USA.

It is centred on a large luminous green emerald surrounded by diamonds as sharp as stars. It is undoubtedly beautiful; designed by Tiffany & Co. in the 1950s, it evokes an era of glamour and grace. But as exceptional a piece of design as it may be, the brooch is little more than a throne for the emerald that it carries.

The Hooker Emerald, named after the Institution’s principal benefactor, Janet Annenberg Hooker, weighs 75.47 carats and was originally extracted from present-day Colombia in the 16th century, when talk of the majesty of these gems had only just begun to travel.

Emerald, Composite Soude, Image Credit: Gem-A

READ MORE: Steve Moriarty on a Lifetime in Gems

The Spanish Conquistadors arrived in the ‘New World’ in the last decade of the 15th century.

When Hernan Cortez was presented with emeralds by the Aztec Emperor Montezuma II in 1519, the allure of the green gem incited the greed of the invaders and a bid to uncover their source was advanced, often leading to violence and the ultimate mistreatment of the indigenous tribes.

While emeralds became symbols of status and wealth at the end of the trade route set up by the Spanish to India and Europe, what did these stones mean to the original inhabitants of the luscious and majestic Andean terrain?

The first of the tribes’ emerald deposits was located by Conquistador, Gonzalo Jimenez de Queseda in 1537 in the village of Somondoco – home to the Muisca (or Chibcha) people.

This deposit would come to be known as ‘Chivor’, meaning ‘our farm fields, our mother’ or ‘green and rich land’ in the native tongue of the Chibcha, a reference to the emeralds un-earthed there.

Lush Colombian Landscape. Image Credit: Pixabay.com

The Muisca people were one of the four principal civilisations of the Americas.

The other three, the Incas, the Mayans and the Aztecs are perhaps more prevalent in Western thought due to the grandeur and ceremony of the architecture that remains as evidence of their complex culture.

But the Muiscas were no less advanced; they were a self-sufficient people existing in comparative isolation in the highlands of the Cordillera Oriental of the Northern Andes.

It is in this Eastern chain of peaks that pockets of the finest emeralds had formed.

READ MORE: Journal Digest, Blue Zircon from Cambodia

The abundance of the precious mineral within the Muisca’s territory, and the ability of the people to mine it efficiently, made it an important economic resource.

Markets were held regularly in conjunction with calendared festivals during which the Muisca would trade the emeralds with gold from the Guane people from north of the Chicamocha River, yopo (a hallucigenic snuff), exotic feathers and jaguar skins from the lowlands, marine snail shells, avocados and the still celebrated ‘ice-cream bean’ from their coastal cousins, the Tairona people.

Emerald in Matrix. Photo Credit: Henry Mesa. 

There is no evidence to suggest that emeralds were ranked higher in value than the other traded goods, but it is apparent that the stone held substantial symbolic weight.

In 1637, the writer Juan Rodriguez Freyle documented an initiation ceremony performed by the Muisca. On the event of a ruler’s death, the successor would be covered with a fine dusting of gold and placed on a raft at the centre of Lake Guatavita.

As music and dancing defined the shores, the new leader would throw gold and emerald votives into the lake as offerings to the Sun God.

This became known as the myth of El Dorado and rumours of a place saturated with such potential material wealth have been, from the time of its discovery to the present day, exploited by the greed of Western adventurers.

This imposition of material desire on the lands of the natives has ultimately led to wars and bloodshed over the centuries.

Emerald in Matrix. Photo Credit: Henry Mesa.

Therefore, is it perhaps worth considering the stance of the Muisca, who did not covet the emerald as their own but accepted it as a spectacular gift from the mountains, and one that they would happily relinquish to maintain harmony with the gods.

If we were to eliminate the profitability of the gem, could we too be able to see deeper into that mesmerising green? Idealistic? Perhaps. I cannot see anyone throwing the Hooker emerald into a lake any time soon. ■

This article originally appeared in Gems&Jewellery Summer 2018/ Volume 27/ No.2 

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: Emerald with Quartz. Image Credit: Henry Mesa.  

---

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

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

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

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

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

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

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

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 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

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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