NDT Solution

Non-destructive Testing for Identifying Natural, Synthetic, Treated, and Imitation Gem Materials

(via Gemmology Queensland, Volume 4, No.9, September 2003) Taijin Lu and James E Shigley writes:

In the daily business of NDT, it is easy to lose sight of the wide range of applications for NDT techniques. In this month’s featured article, the authors have outlined the application of NDT techniques in evaluating natural and synthetic gems. It is always interesting to see in what new ways NDT is used, and how it helps diverse industries or endeavors—G.P.Singh, Associate Technical Editor, The American Society For NonDestructive Testing.

Introduction
Accurate identification is essential to maintain the commercial value of gemstones. It is also required to enable jewelers to disclose to their customers the correct identity of the gemstones they sell. In identifying and studying fashioned gemstones, any testing must be performed nondestructively without altering the gemstones shape or appearance. When presented for testing, some gemstones are mounted in jewelry, which can further complicate the testing procedure. By using standard gem testing instruments and techniques, trained gemologists can recognize most gem materials. In some instances, however, they may not be able to distinguish certain synthetic and laboratory treated gem materials, which can have almost the same visual appearance and gemological properties as their natural counterparts. Use of advanced nondestructive imaging, spectroscopic, and chemical analysis techniques helps establish criteria to distinguish gem materials. Several examples are presented of nondestructive testing techniques currently used in gemology.

Gems include a wide range of materials (natural, synthetic, treated, and imitation diamonds, colored stones, and pearls). This requires gathering observations and measurements to determine distinctive characteristics of gem materials. Such characteristics are summarized in standard reference books (Webster, 1994; Liddicoat, 1981).

With the continued development of crystal growth technology, various synthetic and imitation gem materials are increasingly encountered in the jewelry industry (Nassau, 1980; Nassau, 1983, Gubelin and Koivula, 1986; Nassau et al., 1997). For example, synthetic cubic zirconia, and synthetic moissanite (SiC-6H) have a very similar appearance to diamonds, and are used as imitation diamonds. Some of these synthetic materials, such as synthetic ruby, sapphire and emerald, can be grown by several growth techniques (hydrothermal, flux, melt).

Various processes (such as irradiation, heating, filling of open fractures or cavities, and coating) are also used to treat low quality gem materials to improve their color, appearance, or durability (Moses et al., 1999; Johnson et al., 1999; McClure et al., 1999, Reinitz et al., 2000, Nassau, 1983). Typical examples of heat treated gemstones include sapphire (corundum) and citrine (quartz). Irradiation with gamma rays, X-rays, electrons, or neutrons is used to alter the color of specimens of diamond, topaz, and quartz. Both diamonds and emeralds can have surface reaching openings filled with various substances to improve their clarity.

Since synthetic and treated gem materials are regularly seen today in the jewelry market, it is important for gemologists to be able to identify them. Although effective in many cases, traditional gem testing instruments and methods cannot always distinguish synthetic and laboratory treated gem materials from their natural counterparts. Advanced nondestructive characterization techniques enable the gemologist to find diagnostic properties to distinguish these materials. The goal of the gem research program at the Gemological Institute of America is to find relatively practical methods and/or instruments to identify gemstones based on the differences revealed by both standard and advanced nondestructive characterization. Systematic documentation of the gemological properties of various gem materials of known origin, carried out by similar research institutions and others, is also necessary to develop gem identification criteria.

In this article, characterization methods commonly used today in gemology are briefly reviewed, and examples of several methods to identify gem materials are discussed.

Nondestructive methods of gem identification
A number of instruments have been developed over the years to assist the jeweler and gemologist in the identification of gem materials. These instruments are mainly used for evaluating the appearance of gemstones. Visual characteristics such a color, luster, growth features, and inclusions are examined with a 10x magnifier (loupe) and a binocular microscope with bright and dark field illumination and polarizing functions. Refractive index and birefringence (double refraction) are determined by a refractometer. Pleochroism (dichroism) is examined with a dichroscope. Strains and optical character are observed using a polariscope. Visible absorption spectrum is examined with a hand spectroscope. Specific gravity, electric conductivity, and thermal conductivity are measured by a hydrostatic balance, conductivity meter, and thermal conductivity tester respectively. Trained gemologists are familiar with this equipment (Webster, 1994; Liddicoat, 1981).

Advanced methods
The physical and compositional differences between natural, synthetic, and treated gem materials can range from significant to minor. Thus, it is necessary to check for potentially small differences through advanced analytical methods which have sufficiently high resolution and sensitivity. To achieve better resolution and sensitivity, minor modifications had to be made to some of these instruments. These include the design of special sample holders for faceted and/or mounted gem samples, and optimizing the illumination conditions by use of, for example, an optical fiber system for microscopy and spectroscopy. Special accessories such as liquid nitrogen cryogenic unit are also used to record visible absorption spectra of diamonds at low temperature.

Use of the techniques, together with published information, allows the systematic gathering of a database of information on the gem materials to support the developments of practical gem identification criteria.

Application
In the past, a number of low cost natural minerals and man made materials have been used as imitations of colorless diamond. Since these materials differ compositionally from diamond, they have significant differences in one or more of their physical properties, which allow them to be distinguished. The most widespread diamond imitation material today is cubic zirconia (cubic zirconium oxide). Because diamond’s thermal conductivity is superior to that of imitations, jewelers can quickly distinguish diamonds from cubic zirconia and other previously used imitation materials (synthetic corundum, synthetic spinel, zircon, synthetic rutile, strontium titanate, yttrium aluminum garnet, and gadolium gallium garnet) by use of a simple thermal conductivity meter.

Within the past two years, anew near colorless diamond imitation, synthetic moissanite (SiC-6H), has been marketed for jewelry purposes. The thermal conductivity of synthetic moissanite is quite close to that of diamond, so much so that is can be mistakenly identified as diamond using a conventional thermal probe. However, it is optically anisotropic, which causes the junctions between adjacent polished facets to appear doubled when looking through a fashioned piece with a 10x magnifier or a binocular microscope. The refractive appearance of natural diamonds, on the other hand, always includes clearly single junctions between facets. Synthetic moissanite also exhibits electrical conductivity, and has quite different visible, infrared, and Raman spectra from that of diamonds (Nassau, 1983).

Some natural diamonds possess open, surface reaching cleavages which reflect light, and thus make a fashioned diamond look less attractive. During the past decade, a process has been developed to inject a high refractive index, glass-like material into these cleavages to lessen their visibility. Diamond treated in this way can be recognized by the distinctive visual appearance of the filled cracks that can be seen with 10x magnification. Confirmation of this conclusion can be obtained by X-ray radiography, where the photographic image reveals the filled crack. Chemical analysis can detect the presence of high atomic weight elements such as lead or bismuth, which are components of the glass-like filler material used to increase the refractive index (Koivula et al, 1989; Kammerling et al, 1994).

Coloration can be produced in diamond by exposing them to a source of radiation. This coloration can be modified by subsequent heat treatment. In some cases, irradiation treatment gives rise to unusual color zoning or ultraviolet fluorescence, which can help the gemologist identify a color treated diamond. However, distinguishing colored diamonds irradiated in the laboratory (versus the very rare green to blue diamonds irradiated in nature) normally requires recording visible absorption spectra (with the diamond cooled to low temperatures with cryogenic unit), as well as infrared spectra. In some cases, the distinction cannot be made between the exposure of a diamond to a source of radiation in nature or the laboratory. Recently, several companies have begun using high pressure and high temperature processes to treat the color and other characteristics of a select group of natural diamonds. Conclusive identification of these treated color diamonds requires infrared and low temperature visible spectroscopy as well as Raman spectroscopy, but a number of gemological properties provide some indication of HPHT treatment (Moses et al., 1999; Fisher and Spits, 2000); Reinitz et al, 2000).

Synthetic diamond can be grown from a molten metal alloy at high temperatures and pressures by the temperature gradient. The resulting crystals exhibit a cuboctahedral shape (Sunagawa, 1995). Distinctive color zoning, growth features, and metallic inclusions in polished synthetic diamonds can all be observed with a binocular microscope (Shigley et al., 1995). An ultraviolet fluorescence imaging system, known as the DiamondView and developed by De Beers researchers, allows the observation of the fluorescence zoning pattern that is characteristic of synthetic diamonds (Welbourn, 1996). Chemical analysis can reveal the presence of the transition metals (such as iron, nickel, and cobalt) from which the synthetic diamonds grew.
Natural pearls are relatively rare, but are still encountered in jewelry pieces. Most pearls sold today are cultured pearls. They are produced by introducing a mother-of-pearl bead and piece of mantle tissue into the oyster. The oyster is then cultivated under controlled conditions for periods of up to one year or more during which the bead is covered by nacre. In contrast, pearl imitations are usually wax-filled glass, solid glass, plastic, and mother-of-pearl (Liddicoat, 1981; Webster, 1994).

X-radiography is the best method to identify pearls. X-ray absorption images of pearls are recorded on photographic film to reveal the differing internal structures (Liddicoat, 1981; Matlins, 1995; Scarratt et al., 2000). On a radiograph, natural pearls usually show concentric rings outward from the center. Cultured pearls show a clear interface between nacre layer and the bead nucleus, while pearl imitations lack internal structure.

Cultured pearls can be colored by chemical treatments or irradiation. They may also possess a surface coating. Such treatments can often be detected by careful examination with a binocular microscope; chemical analysis may also reveal the presence of a foreign material.

Jade is a polycrystalline gem material consisting of a mixture of silicate minerals. There are two types of jade, one consisting mainly of jadeite (a pyroxene), and the other of nephrite (an amphibole). There are also other green-colored rocks that have been sold as jade, as well as imitations. Since there are a large number of grain boundaries and/or fractures in jade, several treatment processes are used to improve appearance. The most commonly used processes involve chemical bleaching of the original jade to remove brownish stains, and then impregnating it with polymers or other substances to fill surface reaching fractures (Fritsch et al., 1992; Ou Yang, 1997). In some cases, colored dyes are also used to improve the color.

A number of identification criteria for treated jade have been found, such as the peculiarities evident under magnified observation, lower specific gravity, and distinctive infrared spectra. When observed under magnification, it is often possible to see differences in texture or structure that are evidence of treatment. Since the specific gravity of untreated jade is around 3.3 to 3.5, polymer impregnated samples will float in a 3.2 specific gravity liquid because of their lighter specific gravity, whereas untreated jade will sink. Infrared absorption spectra show clear differences between natural and treated jadeite.

Ruby (corundum) is not only one of the best known colored gemstones, but it is also an important optical crystal for industrial uses. It can be synthesized by many methods, such as from a melt by the Czochralski technique, from a flux solution, and recently from a hydrothermal solution (Lu and Shigley, 1998). Synthetic ruby can often be distinguished gemologically by the observation of inclusions, color zoning, and growth features (Liddicoat, 1981; Hughes, 1997, Lu and Shigley, 1998).

When such visual characteristics are not present or cannot be seen, natural and synthetic rubies can be identified on the basis of their trace element chemistry as determined by the energy dispersive X-ray fluorescence method (Muhlmeister et al.,1998). Trace elements such as nickel, molybdenum, lanthanum, tungsten, platinum, lead, or bismuth may be detected in synthetic ruby, as a result of growth in a laboratory environment. The relative proportions of the transition metals chromium, iron, vanadium, and titanium also varies between natural and synthetic rubies, and can be used to distinguish between them.

Amethyst (quartz) is the most important gem variety of quartz, and is found in many localities worldwide. Its synthetic counterpart, grown from either alkaline or fluoride solutions, is produced in Russia, Japan, China, and other countries. The appearance and physical properties of both are very similar.

Based on our study of both natural and synthetic amethyst, we have found that there are characteristic differences in crystal morphology, twinning, inclusions, growth zoning, color banding, infrared spectra, and trace element chemistry. As noted by Lu and Sunagawa (1990), natural amethyst is usually twinned, it exhibits tiny solid inclusions, and it lacks a characteristic infrared absorption band at 2.8mm (1x10¯4 in). Synthetic amethyst grown from fluoride solution displays an unusual growth structure (a stream-like structure), color bands that parallel the seed crystal, distinct infrared spectra, and fluorine and lithium as trace elements. The synthetic amethyst grown from alkaline solution is characterized by growth sectors, color zoning, and distinct infrared spectra.

Emerald (beryl) is the final example of gem identification to be discussed here. Synthetic emerald can be grown from flux and hydrothermal solutions, and it exhibits a number of gemological properties (like synthetic ruby) by which it can be identified.

Many natural emerald crystals often contain surface reaching fractures, due to their growth conditions or perhaps to damage suffered during mining. The presence of these fractures results in an unattractive appearance. Increasingly over the past two decades, a variety of liquid substances (oils, epoxies, resins, etc.) have been used to fill these open fractures, and thus to improve emerald appearance (Kammerling et al, 1991; Johnson et al., 1999). Questions concerning the long term stability of these substances have led to the need not only to determine if an emerald has been treated in this way, but to determine the identity of the filler substance as well. The use of both infrared and Raman spectroscopies has provided new information on the detection of treated emeralds (Johnson et al., 1999).

Discussion and summary
Gem materials consist of a wide range of materials. Most of them are single crystals, usually faceted and polished (and sometimes mounted in jewelry). To support the commercial value of gemstones and the stability of the jewelry market, accurate gem identification is important. The traditional and advanced nondestructive characterization techniques used today for gem identification are based on the distinctive gemological properties of natural, synthetic, treated and imitation gem materials.

Traditional gemological methods and instruments are inexpensive and relatively easy to use. When used by a trained gemologist, they can help to identify most gem materials. In some cases, advanced analytical techniques with higher resolution and sensitivity are required to reveal the small differences in properties among natural, synthetic, and treated gem materials. Advanced techniques can also help identify the geological environment and growth conditions in which a gem material formed. They can also occasionally provide evidence on the geographic origin, which is important for high value natural colored stones such as ruby or sapphire. Although a combination of both the traditional and advanced methods helps to determine the diagnostic properties of gem materials, the development of new, relatively simple and inexpensive testing instruments for use by jewelers remains a challenge. The continued production of new synthetic and treated gem materials requires an equal effort to support practical gem identification to safeguard the jewelry industry.