Gemmology Canada Special Edition
by Ray Penner, M.Sc., A.G. (C.I.G.)
Microscopic examination of characteristic inclusions is the traditional method used to distinguish between natural emeralds and synthetic emeralds. This method of separation is most difficult, especially for the inexperienced gemmologist, as a result of the similarities of some of the inclusions found in both natural and synthetic emeralds. This difficulty is compounded even further when one is dealing with near flawless stones. It is therefore very important that other classical gemmological methods are used in conjunction with the microscope to aid in the identification of synthetic emeralds. The use of refractive indices, specific gravity, UV fluorescence, and the Chelsea filter in the identification of synthetic emeralds will be overviewed in this paper.
In some cases classical gemmological methods will not provide enough clues to make a positive identification and more sophisticated techniques may be necessary. These techniques typically involve equipment that is generally too expen sive for practical ownership by most gemmologists. However a gemmologist still needs to keep abreast of these techniques which are being more and more commonly used by the more advanced gemmological laboratories throughout the world. The techniques to be discussed are infra-red spectroscopy, chemical analysis with the microprobe and by X-ray fluorescence, cathode luminescence and thermal conductivity. Hopefully some of these techniques may be available to more gemmologists in the near future.
Studies of natural emeralds have shown that both the filling of structural voids and the substitution of Cr, Fe, Mg, Li, other ions, and water molecules appear to be the major cause of variations in the refractive indices. The refractive indices of natural emeralds range from (e=1.569, o=1.576) for certain Columbian emeralds to ( e= 1. 592 , o= 1. 602 ) for certain Zambian emeralds . Just as with natural emeralds , the refractive indices of synthetic emeralds are also dependent in part on the amount of impurity ions and molecules they contain. As a result of the different synthesis techniques used by different manufacturers these amounts frequently differ.
Flux-grown synthetic emeralds have significantly different refractive indices as compared with natural emeralds. A typical example is a Chatham synthetic emerald whose values are (e=1.560, o=1.565). Careful measurement of refractive indices will quickly distinguish flux-grown synthetics from natural emeralds. In addition the typical birefringe of flux-grown synthetics is .004 while those of natural emeralds are .006 and higher - though in this regard there is some overlapping.
Hydro-thermal synthetic emeralds are a different matter. In most cases,their refractive indices are only slightly lower than their natural counterparts, examples include Inamori (e=1.563, o=1.568) and Biron (e=1.569, o=1.573), and although an identification should not be based solely on their property, these values are unlikely to be observed in a natural emerald. Unfortunately some hydro-thermal emeralds, specifically some of the emeralds produced by Lechleitner, overlap the refractive indices of natural emeralds.
As with refractive indices the specific gravity values of flux-grown synthetic emeralds are typically lower than the values of their natural counterparts . At the low end for natural emeralds certain stones from Zambia and Brazil have a specific gravity of 2 .68 while the typical emerald from Columbia has specific gravity of 2 .70. The values for the flux-grown synthetics range from 2.62 to 2.66 and thus can be separated from natural emeralds using a standard 2.67 heavy liquid. Exceptions are certain Gilson flux-grown synthetics whose values can reach 2 .70 and heavily flawed natural stones whose specific value may be lower than 2.68.
The specific gravity values obtained for most hydro-thermal synthetics are slightly higher than their flux-grown cousins with values ranging from 2.67 to 2.71. As their values overlap the specific gravity range of natural emeralds a specific gravity test will not distinguish them. An exception are certain Inamori synthetics whose values can be as low as 2.65.
An inert reaction to ultraviolet radiation has frequently being used as an indicator of natural origin for emeralds, since the majority of natural emeralds are inert to either longwave or shortwave UV. It is the presence of iron in these natural stones which quenches the chromium fluorescence. There are exceptions however as some high-chromium Columbian emeralds will fluoresce bright red under long-wave UV radiation. In addition natural emeralds which have been oiled (synthetics may also be oiled) will show yellow fluorescing patches or veins when viewed under the UV lamp. It is important that the test is performed in a completely darkened room with the emeralds placed on a black pad within a few inches of the ultraviolet lamp.
Many synthetics, both flux-grown and hydro-thermal, show a strong reaction to the ultraviolet lamp. These include synthetic emeralds produced by Linde, Lenix, and Chatham, all of which fluoresce a strong red under longwave UV-rays. Stones produced by Gilson have a yellowish luminescence under long-wave UV rays and orange luminescence under shortwave UV-rays. Stones produced by Seiko have a distinctive green fluorescence under long-wave UV. Unfortunately for the gemmologist several types of synthetics are inert, as with the majority of natural stones, to UV rays. Specifically synthetic emeralds produced by Biron (their lack of fluorescence is attributed to the high concentration of vanadium which acts like iron to quench any fluorescence) and Crystal research along with the Russian hydro-thermal synthetics, which are doped with iron, are inert to UV-radiation.
To summarize, though strong fluorescence is usually indicative of synthetic origin the absence of fluorescence cannot be used as an identifying criterion.
Natural stones typically have a weak red appearance under the Chelsea filter though stones from various localities will cover the whole range, from no reaction to a bright red. In the case of synthetic emeralds there is a tendency towards the bright red; good examples are stones by Lenix and Linde, though once again the whole range is covered with: for example Russian hydro-thermal synthetics which show no reaction under the Chelsea filter. Thus a strong red appearance under the Chelsea filter is only weakly indicative of synthetic origin for an emerald.
Infra-red spectroscopy has in the recent years become an important identification tool used by gemmological labs. The infra-red spectrum is as the name applies next to the red end of the visible spectrum. The wavelength ranges, from 750 nm (corresponding to the limits of the visible spectrum) to approximately 300,000 nm. For most gemmological purposes the range which is of greatest interest extends only to 25,000 nm. Just as with optical spectroscopy, infra-red spectroscopy involves the observation, or more correctly the recording, of the absorption spectrum of the specimen. A fundamental difference between the two is that while optical spectroscopy involves the transition of electrons in atoms or molecules, infra-red spectroscopy involves transitions between vibrational states of molecular and structural components of the crystal.
The most important use of infra-red spectroscopy with respect to emeralds is in the detection of water in the sample stone. The incorporation of water during growth in both natural stones and hydro-thermal synthetic emeralds causes absorp tion between 2,500 nm and 3,000 nm in the infra-red spectrum, whereas the flux material uses no water so the corresponding synthetics lack absorption in this region. Thus the detection of flux synthetic emeralds is straight forward with the use of infra-red spectroscopy.
The detection of hydro-thermal synthetic emeralds is a different matter as their infra-red spectra are more similar to that of a natural stone. It has been found, however, that the majority of hydro-thermal synthetic emeralds exhibit a pattern of strong absorption in the infra-red between 3300 nm and 3850 nm which will dis tinguish them from natural emeralds. Unfortunately this does not apply to all hydro-thermal synthetic emeralds as the infra-red spectrum of the Russian hy drothermal product is extremely similar to natural emeralds. There are slight differences which enable gemmologists to distinguish between natural emeralds and the Russian hydro-thermal products but these are apparent only with a good infra-red spectrometer.
Microprobe Chemical Analysis
It has been found that chemical differences exist between synthetic and natural gem materials. Natural emeralds incorporate a variety of nonessential chemical components from the natural environments in which they are formed that are not present in their synthetic counterparts. Conversely elements are frequently in corporated into the artificial environments created for the growth of synthetic crystals which are not found in their natural counterparts.
The chemical composition of emeralds, and most other gem materials, can be determined with an electron microprobe. With the electron microprobe a narrow beam of energetic electrons are focused on a small point on the surface of the stone. Analysis of the resulting X-ray radiation allows the determination of the chemical elements present in the specimen.
The most significant difference found is that chlorine has been found in all tested hydro-thermal synthetics but in no other emeralds. It apparently comes from the chlorine hydrate that is used to supply chromium as a coloring agent in hydro-thermal synthetics. Several other components are found in general to be present in greater quantities in natural emeralds than in synthetics.
X-Ray Fluorescence (XRF) analysis is a sophisticated technique which is used, as with the electron microprobe, to determine which elements are present in a given specimen. The idea is the same as with the electron microprobe except that in an X-RF spectrometer an X-Ray beam, instead of an electron beam, is directed at the sample which causes the individual chemical elements to emit X-rays. Elements which are not present in quantities great enough to be detected by the micro probe can be detected with XRF. Results have shown that several elements, examples include potassium and calcium, identified at trace levels in many of the natural emeralds were not found at corresponding levels in any tested synthetic emeralds. In addition the element rhodium was found in both flux and hydro-thermal synthetics but not in any tested natural stone.
Cathodluminescence is the emission of visible light by a material excited with an electron beam. The difference between this and the electron microprobe is that a lower energy electron beam is used and it is the emission of visible light and not X-rays which is of interest. In general synthetic emeralds typically have a red luminescence which is stronger than their natural counter- parts. Lennix synthetic emeralds, unlike either natural or other synthetics, have been found to display purple or bright-blue luminescence.
Tests carried out on both synthetic and natural emeralds have shown that, in most instances, thermal conductivity readings from synthetic emeralds are significantly lower than those from most natural emeralds.
Conclusion To be able to distinguish between synthetic and natural gemstones is crucial for a gemmologist. Skill with the microscope in observing tell-tale inclusions is still the dominant method used in the gemmological community. In the case of emeralds however, classical gemmological tests such as the measurement of refractive indices, specific gravity, as well as reaction to UV light and the Chelsea filter will often distinguish between natural and synthetic stones. These tests should always be carried out in conjunction with the microscopic inspection.
New techniques such as infrared spectroscopy and chemical analysis as well as others are further tools which are able to distinguish between natural and synthetic stones. As synthetics continue to become more sophisticated the range of tools required to identify them will continue to expand. Hopefully the gemmological tools will be able to keep pace with the sophistication of the synthetics.
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Ray Penner is a physics instructor at Malaspina College in Nanaimo. This paper was submitted in fulfillment of his "Accredited Gemmologist (C.I.G.)" diploma requirement.
This Special Issue of Gemmology Canada is published for students of the Canadian Institute of Gemmology and others interested in gemmology. The copyright for any articles remains with the author. For further information write to the C.I.G., P.O. Box 57010, Vancouver, B.C. V5K 5G6 or phone/fax (604) 5308569 or email firstname.lastname@example.org
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