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Focus on Gemstone Fluorescence: Looking for the Light

Published in Gem Knowledge

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.

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.

Colourless Gemstones	LWUV	SWUV
Diamond (natural)	Strongest reaction, most common is blue, but can be yellow and green	Similar colours to LWUV but it is a weaker reaction
Diamond (synthetic)	Similar colours to SWUV but it is a weaker reaction	Strongest reaction, mainly fluoresce orange to yellow
Cubic zirconia	Weak to inert (usually inert) but same colours as SWUV, yellowish-orange.	Moderate to strong yellowish-orange (apricot), variable
Synthetic moissanite	Variable reactions	Variable reactions
Synthetic spinel	Inert	Bright, chalky white or blue/green
Paste	Inert	Variable, may have chalky white surface

Red and Pink Gemstones	LWUV	SWUV
Ruby (natural)	Variable, strong red to inert	Same as LWUV but weaker to inert reaction
Ruby (synthetic)	Bright red, tends to be stronger than natural ruby	Red, weaker than LWUV but still brighter than natural ruby
Red spinel	Red	Red, but weaker than LWUV
Spodumene, var. Kunzite	Orange or violet	Weaker violet, whitish or inert
Almandine garnet (Iron is present, this stone never fluoresces)	Inert	Inert
Red glass/paste	Inert	Variable, may have chalky white surface

Yellow Gemstones	LWUV	SWUV
Quartz, variety cirtrine	Inert	Inert
Yellow sapphire (natural)	Apricot orange to inert	Inert
Yellow sapphite (synthetic)	Weak red to inert	Inert
Yellow scapolite	Yellowish	Reddish
Yellow topaz	Yellowish	Whitish

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.

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Investigating Fake Rough with Gem Testing Laboratory Jaipur

Published in Gem Knowledge

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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