Identifying faceted gemstones involves practices that are closely related to

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1 The Identification of Faceted Gemstones: From the Naked Eye to Laboratory Techniques ertrand Devouard 1 and Franck Notari /09/ $2.50 DOI: /gselements Identifying faceted gemstones involves practices that are closely related to the classical determinative methods used by mineralogists. Measurements of optical and physical properties, combined with acute observation using various illumination techniques, are usually sufficient to determine the nature of a gem. Determining the geographic origin of a gem or the enhancement treatments it was subjected to, however, can require the expertise of an experienced gemologist and a combination of spectroscopic laboratory techniques. Ke y w o r d s : gems, gemology, optical properties, inclusions, spectroscopy INTRODUCTION Identifying a faceted gemstone first implies determining the material of which it is made (the terminology used in this article is illustrated in Fig u r e 1). Then, the challenge is to determine if the stone is natural or synthetic (Fritsch and Rondeau 2009 this issue; Kane 2009 this issue) and, most importantly, if the stone has undergone one or more enhancement treatments. Gems are frequently subjected to various treatments in order to improve their appearance in terms of color and transparency, and hence increase their commercial value (Nassau 1983). The gemologist may also attempt to determine the geographic origin of the stone (or, possibly, the method of synthesis) since the origin, when it can be assessed, may influence the stone s market value. Mineralogists and petrologists routinely identify minerals with a variety of simple or sophisticated methods. Gemologists use similar techniques, but identifying gemstones differs from identifying minerals in a rock for obvious reasons: destructive methods and those that visibly alter the stone are proscribed (hardness tests are typically not an option!). Moreover, gems are sometimes set in jewelry and cannot be unmounted, seriously limiting the scope of certain techniques. Professional gemologists favor simple, quick, and inexpensive techniques, which are indeed sufficient in many cases for species and variety identification. Think of identifying hundreds of tiny stones in a pavé setting: efficiency is of the essence. In some cases, however, observation and simple techniques will fail to provide answers (e.g. the detection of certain treatments and the stone s geographical origin); then, laboratory methods are required. In the present paper, we summarize the main methods of identification used in gemology, from the basic tools to so-called advanced analytical methods, and we illustrate these with specific examples of problems encountered in the characterization of gemstones. SIMPLE TOOLS FOR ASIC PROPERTIES Optical Properties Just as a mineralogist uses color, relief, interference colors, and conoscopic observation to identify minerals in a rock thin section examined under a polarizing microscope, the gemologist observes the purity and color of a stone and estimates its refractive index from its luster and the dispersion index from the fire colors in light-colored stones. Color is of course a most important property of gemstones. Estimating and quantifying colors is a difficult task and will not be discussed here (but see Hofer 1998). With a handheld spectroscope, one can observe absorption or emission bands in the visible spectrum, which can help identification. Pleochroism of gemstones, when strong enough, can be observed with the naked eye. It can also be seen using a polarizing filter or, even better, a dichroscope, a small handheld optical instrument allowing a fine comparison of pleochroic colors by juxtaposing the images of the different rays (Fig. 2). table culet pavilion crown girdle 1 Laboratoire Magmas et Volcans (UMR 6524) Université laise Pascal CNRS, 5 rue Kessler F Clermont-Ferrand, France .Devouard@opgc.univ-bpclermont.fr 2 GemTechLab Laboratory, 4 bis route des Jeunes CH 1227 Acacias, Geneva, Switzerland franck.notari@gemtechlab.ch Figure 1 Outline of a faceted gem (brilliant cut, side view) showing the terms used in this paper. Faceting allows light entering the stone from above (e.g. path in red) to be refracted and reflected so that rays are directed back to the observer, thus enhancing the esthetics of the gem. Proportions and angles have to be adjusted to the optical properties of the material. Computer simulations show that not only refractive indices, but also more subtle phenomena such as polarization on reflection, influence the visual aspect of a faceted stone (Moses et al. 2004). El e m e n t s, Vo l. 5, p p

2 A The refractive index is a key property for identifying a gemstone. It can be efficiently measured to the second decimal place with a refractometer, a simple instrument exploiting total reflection of light on a facet (usually the table) of a cut gemstone. Skilled gemologists can also determine whether the stone is anisotropic, measure the two indices (birefringence) in the section corresponding to the facet (and repeat the measurement for various orientations, if the stone is large enough), and even determine the uniaxial or biaxial character and the optical sign. Another easy way to estimate the birefringence of a gemstone is with a magnifying loupe or a binocular microscope. The gemologist looks through the stone s table to observe if the edges of the pavilion facets appear to be doubled (Fig. 3). As with a calcite rhomb, anisotropic materials will show doubling, the effect being more or less pronounced depending on the value of the birefringence, the direction of observation, and the length of the optical path inside the gem. Figure 2 Tanzanite, a variety of zoisite, is an attractive gem with spectacular pleochroism. (A) Pleochroism in an unheated tanzanite, observed in immersion with one polarizing filter set under the specimen. In such conditions one can observe three colors: purple, blue, and yellow, corresponding to α, β, and γ rays, respectively. The stone pictured weighs 0.45 carat. () The three rays, polarized at right angles, can be observed in pairs with a dichroscope, depending on the observation direction. Heat treatment of tanzanite changes the color of the γ ray from yellow to blue, resulting in stones with an intense blue color when the table of the stone is cut parallel to the (100) face of the rough crystal, or a purplish blue color if the table is cut perpendicular to (001). Picture F. Notari The polariscope is another simple tool, which makes use of crossed polarizing filters and a source of light. This instrument separates optically isotropic from anisotropic gemstones. With a glass bead stuck on a small handle (conoscope), it allows conoscopic measurements as with a polarizing microscope. In addition to measuring optical properties (e.g. 2V angles) and detecting anomalous double refringence (strain), the polariscope and conoscope also serve to orient gemstones prior to spectroscopic measurements. These simple tools, combined with careful observation, are often sufficient to unambiguously identify the mineral species of a gemstone. For instance, a dichroscope and a handheld spectroscope would reveal whether the lack Prince s ruby (a 170 carat red stone set in the front of the imperial state crown of the United Kingdom) is a ruby or a red spinel (it actually is a spinel). For a more detailed description of the basic tools of gemology, refer to Webster and Read (1994). Other Physical Properties Other basic properties are useful for identifying gemstones. The measurement of specific gravity using heavy liquids or a hydrostatic scale gives very precise results on inclusionfree gemstones. As an example, it has been shown that emeralds synthesized using flux methods can be distinguished from natural or hydrothermally synthesized emeralds by their specific gravity (S.G.), because the channels in the structure of the flux-grown crystals are empty (giving S.G. = ), whereas they contain water and alkali ions in natural or hydrothermally synthesized stones (S.G. > 2.68). Magnetism is another physical property that can be evaluated with simple testing, for example, by floating a stone on a small piece of polystyrene foam placed on water and submitting it to the magnetic field of a strong magnet. Paramagnetic (iron-containing) minerals can be detected this way. The method can also be applied to identify synthetic diamonds grown in metal flux. These diamonds are often weakly magnetic because they contain impurities from the flux. Estimating thermal conductivity can also aid in gem identification. Stories are told of gemologists able to recognize glass from quartz or brown topaz from low-priced citrine with their eyes closed, just by estimating the thermal conductivity from the sensation of cold (for quartz) when holding the stones against their lips. Thermal conductivity is the property used in diamond tester instruments intended to distinguish diamonds from their simulants. Although popular, diamond testers are not foolproof. The A C Figure 3 irefringence can be estimated by observing the doubling of edges between facets in the pavilion and culet while looking through the table or through the crown of a faceted gem. (A) Diamond, isotropic, shows no doubling. Width of field ca. 1.6 mm. () Moissanite (SiC, 6H polytype), a convincing simulant of diamond with a birefringence of 0.036, shows a typical fuzzy aspect due to moderate doubling of edges, and it can be distinguished from diamond in this way. Field of view ca. 1.6 mm. (C) Zircon, with a high birefringence (0.055, when not metamict), exhibits strong doubling. Field of view ca. 5 mm. Estimating the birefringence with this method implies taking into account the thickness and crystallographic orientation of the stone. Photos F. Notari El e m e n t s 164

3 older instruments cannot make the distinction between diamond and synthetic moissanite, a convincing simulant that appeared on the market in the late 1990s (Nassau 2000) and that has a high thermal conductivity, close to that of diamond. Synthetic moissanite, however, can be recognized using other simple techniques, such as observing its birefringence with a loupe (Fig. 3b). OSERVATION IS KEY If the measurement of physical properties with simple tools allows one to determine the mineral species, it usually does not reveal if the stone is natural or synthetic, if it has been treated, or its geographic origin. In most cases, however, these questions can be addressed by careful observation using a variety of illumination techniques at various wavelengths. The naked eye or a loupe might be sufficient for an experienced gemologist, but more reliable observations are made with a binocular microscope (Fig. 4). In some cases it might be necessary to observe a stone in an immersion cell containing a liquid with a matching index of refraction, in order to reduce the disturbing refraction effects on the various facets of the stone. As observation is a very simple technique, it is sometimes felt that it is not great science. Even if not very sophisticated, observation is as valid, robust, and efficient a scientific method as any other. Illumination and Luminescence Techniques When observing gemstones under the binocular microscope, illumination is critical. Transmitted light; apical, oblique, or lateral illumination with concentrated or diffused light; and dark-field imaging can reveal different features. For example, natural and synthetic amethysts can be distinguished by the presence or absence of twinning, as well as its nature, when observed with transmitted light in immersion liquid with the use of crossed polarizing filters. (Fig. 5) (Crowningshield et al. 1986; Notari et al. 2001). A Figure 4 In modern gemology laboratories, visual observation is important. At GemTechLab (Geneva), roughly one-half of the room is occupied by binocular microscopes (A), while the other half () contains analytical instruments such as ED-XRF, FT-Raman, FTIR, and UV-NIR spectrometers. Some gemstones display different colors depending upon the color temperature of the light source (Liu et al. 1994). Such color-change gems, of which alexandrite is the archetype (reddish under incandescent light and greenish under natural light), are eagerly sought by gem collectors. As an alternative to polychromatic visible light, an ultraviolet (UV) or a monochromatic source of light can be used to observe luminescence colors and heterogeneity in a stone. Although luminescence (i.e. fluorescence, phosphorescence, or both) can sometimes be observed with a simple UV lamp (Robbins 1994), special instruments have been devised for the observation of luminescence in gems under the binocular microscope. Diamond View (TM), developed by De eers Diamond Trading Co., is equipped with a UV source at about 220 nm, whereas the U-Visio, developed at GemTechLab, uses various intense wavelengths for excitation from 365 to 500 nm (mainly nm). This allows observation in specific regions of the spectrum, after filtering out the excitation wavelengths and undesirable emissions, e.g. the Cr 3+ red fluorescence in rubies. Figure 5 Growth features can assist in distinguishing natural from synthetic gems. The interference fringes (rewster fringes) observable in this 5.60-carat amethyst lie in the major rhombohedron (r) planes and are caused by polysynthetic twinning (razil twin law). These fringes, visible when observing the stone parallel to the optic c-axis between crossed polarizing filters, are robust evidence for the natural origin of amethyst. Co u r t e s y Th o m a s Ha i ns c h wa n g, Ge m l a b l a b o r at o r y, Li ch t e ns t e i n As an example, a variety of luminescence colors can be exhibited by diamonds. They are induced by impurities and defects. About a third of all gem diamonds luminesce. Most commonly, they display a blue luminescence under long-wave UV light caused by N3 centers (clusters of three atoms of nitrogen replacing three carbon atoms around a vacancy; Woods 1984). Other types of defects in diamond can, however, induce luminescence in other colors, such as the spectacular chartreuse (yellowish green) luminescence induced by H3 centers (the association of a pair of nitrogen atoms an A aggregate with a vacancy), which can usually be observed under intense white-light excitation. Luminescence in diamond is a powerful method for distinguishing natural from synthetic diamonds and for identifying certain treatments (se e Fig. 6). In several cases, orangey fluorescence can indicate e-diffusion treatment in corundum. For detailed investigations, the visual observation of luminescence has to be replaced by spectroscopic analysis of the emitted light (Shigley et al. 1993; Eaton- Magaña et al. 2007). El e m e n t s 165

4 A C Figure 6 Growth patterns, revealed by luminescence, can help to distinguish natural and irradiated diamonds from synthetic stones. (A) The irregular zoning of this apparently black diamond of 1.27 carats is typical of a natural stone. In addition, the sharp luminescence concentrated at the edges of the faceted stone is evidence for treatment by neutron irradiation. () Irregular luminescence patterns are spectacular in this complex 0.20-carat natural brown diamond from Argyle (Australia), which displays a central zone (also refracted by the crown facets) with green luminescence, typical of so-called CO 2 -rich diamonds, surrounded by an overgrowth of type Ia diamond with typical blue luminescence. (C) y contrast, this 0.22-carat synthetic yellow diamond shows green luminescence typical of diamonds grown at HP HT in a metal flux: straight patterns, with sector zoning marked by the traces of {111} faces and weak oscillatory zoning along {100} faces. Such yellowish green luminescence, linked to Ni N defects, is never observed with such a distribution in natural colorless or yellow diamonds. The Internal World of Gemstones Most gemstones, even those of high clarity, contain inclusions. These inclusions, solid (minerals or melts) or fluid, tell the story of the stone s genesis and can be indicative of a geological context (Groat and Laurs 2009 this issue) or even a specific deposit. For synthetic stones, inclusions of flux, platelets, or bubbles in flame-fusion (Verneuil method) corundum or spinel (the latter often displaying typical spindle-shape bubbles) are telltale signs of their method of synthesis. The variety of inclusions in gemstones and their use in identification are described in numerous articles and books, including the reference works by Gübelin and Koivula (1986, 2005, 2008), which contain thousands of photographs. The inclusions of U-rich thorianite in Fig u r e 7a are known in corundum from mainly three deposits (Mogok in Myanmar, Kashmir, and Andranondambo in Madagascar). They are readily identified under U-Visio observation by the fluorescent halo surrounding each of them (Fig. 7b). Combined with other evidence (such as other mineral inclusions, trace element chemistry, and UV visible spectroscopy), such inclusions can help to determine the geographic origin of the stone. The detection of heat treatment in corundum is currently a major issue in the gem trade (Themelis 1992; Emmett 1999). Low-quality metamorphic corundum crystals are routinely treated at high temperature (HT) in order to improve their clarity and/or color (see Fig. 6 in Fritsch and Rondeau 2009 this issue). If the heat treatment is carried out in a reducing atmosphere, the treated crystals can incorporate Ti from rutile (TiO 2 ) inclusions into the corundum structure (the color of blue sapphires is in part due to Fe 2+ Ti 4+ charge transfer). It is possible to obtain the inverse effect (lightening a blue color that is too dark) by heating in an oxidizing atmosphere. Fig u r e 7c shows inclusions in an untreated blue sapphire, as indicated by the presence of fine rutile needles (called silks ) and böhmite. For comparison, Figure 7d shows an uneven distribution of color in a heat-treated sapphire, clearly revealing ghosts of former rutile needles. However, the interpretation of rutile morphologies by microscopic observation requires careful examination and some knowledge of crystal morphology and orientation. Dotted alignments of inclusions in sapphire might represent partially dissolved rutile needles after heat treatment at high T (>1600 C), but they can also be alignments of polycrystalline böhmite, which are not indicative of HT treatment. Unambiguously identifying these two kinds of inclusions in corundum requires determining their orientation relative to the c-axis (optic axis) of the host: rutile inclusions are always distributed in the (0001) plane and cross each other at an angle of 60, whereas böhmite inclusions align parallel to the < 1 231> directions (junctions of the rhombohedron faces). A second type of HT treatment in corundum incorporates the coloring elements by diffusion from the outside (see Shigley and McClure 2009 this issue). Recently, a new type of diffusion-treated sapphire appeared on the market, characterized by very attractive pinkish orange ( padparadscha variety) to deep orange colors. This treatment was identified as diffusion of e 2+ (Emmett et al. 2003). Fi g u r e 7e shows the original reddish color of the stone, the yelloworange zones due to e diffusion, and blue Ti-diffusion halos around prismatic rutile inclusions. In addition to the evidence for HT treatment and beryllium diffusion, the stubby rutile inclusions are characteristic of the Songea (Tanzania) deposit. Crystal Growth as Revealed by Color and Luminescence As crystals grow, variations in their environment can be recorded by the heterogeneous distribution of trace elements or defects. Oscillatory or sector zoning are often observed in colored gemstones. Natural stones display very different zoning from synthetic crystals grown in a dissimilar, better-controlled environment. As a result, zoning patterns can often be used to distinguish natural from synthetic stones. When heterogeneities of color are not observed, growth patterns can sometimes be revealed by luminescence (Fig. 6 and Fig. 7b). Fig u r e 6 shows typical growth patterns displayed by natural and synthetic diamonds. While natural stones often show complex zoning (Fig. 6a, 6b), flux-grown synthetic diamonds display very regular sector zoning, very little oscillatory zoning, and a green fluorescence caused by Ni impurities coming from the metal flux (Fig. 6c). In addition, strong luminescence of the edges between facets (Fig. 6a) is proof of treatment by irradiation (oillat et al. 2001). EXTENDING THE EYE WITH SPECTROSCOPY When simple procedures fail to provide an unambiguous diagnosis, gemology laboratories resort to spectroscopic methods. Examples of situations requiring spectroscopy are the identification of HT treatment subsequent to irradiation in yellow diamonds and the identification of synthetic or heat-treated, natural, inclusion-free, colored El e m e n t s 166

5 A C stones. Large colored stones without any inclusions or heterogeneity of color are always suspicious since they are likely to be synthetic, but on the other hand they command high value if they are natural. D E From Ultraviolet to Infrared Visible light optical spectroscopy quantifies what the eye sees as color and, in favorable cases, assists in identifying the origin of color (Fritsch and Rossman 1987, 1988; Rossman et al. 1991). UV visible spectrometers or Raman spectrometers can also be used for photoluminescence spectroscopy (Chalain et al. 1999). In red spinel, for example, the Cr 3+ emission bands do not exhibit the same pattern in natural as compared to synthetic samples (Notari and Grobon 2003). Spectroscopy is most interesting to investigate domains of the electromagnetic spectrum that are not detected by the human eye, such as UV and IR. Vibrational spectroscopies can be used as fingerprint methods for nondestructive identification of species or varieties (e.g. opal-a from opal- CT), particularly when other gemological properties are very similar. Raman spectroscopy and IR specular reflectance in the cm -1 range (Hainschwang and Notari 2008) are particularly helpful for identifying unusual gems. For instance, these methods can distinguish between the rare gems musgravite (magnesiotaaffeite- 6N 3S) and taaffeite (magnesiotaaffeite-2n 2S), which are otherwise nearly indistinguishable except using X-ray diffraction. Micro-Raman spectroscopy is another common method for determining the nature of inclusions deep inside gemstones. IR spectroscopy is routinely applied to detect trace amounts of water and the presence of organic compounds, and to characterize diamond. For diamond, this technique provides the type and speciation of impurities (N, H, ) and reveals many small, sharp, absorption features related to the treated or synthetic nature of the gem (Zaitsev 2001). As most gems are inorganic, the presence of organic molecules can be proof of impregnation with a resin, oil, or polymer. Trace amounts of water absorb IR differently in some natural gems compared with corresponding hydrothermal synthetics, and this feature can reveal the presence or absence of heat treatment in inclusion-free corundum. In corundum, the absorption band at 3309 cm -1, accompanied by four satellite bands (at 3376, 3295, 3232, and 3187 cm -1 ), is due to OH dipoles linked to Ti or Fe Ti pairs. These features are observed in gems that have undergone an HT event. The observation of these OH bands is thus robust evidence of heat treatment in natural corundum of metamorphic origin but has to be considered with other data (such as trace element content or visible spectroscopy) that can provide relevant information on the geological context. Figure 7 Inclusions in gemstones are often typical of mineral species, geological setting, method of synthesis, or treatment. (A, ) Micrographs of a yellow, unheated sapphire from Mogok, Myanmar, showing fluid, calcite, and U-rich thorianite [(Th,U)O 2 ] inclusions. Field of view ca. 1.8 mm. Transmitted light (A) and corresponding image under U-Visio luminescence (), showing a reddish luminescent background due to traces of Cr 3+, a yellow luminescent zoning due to color centers associated with traces of Mg 2+ (invisible in transmitted light), and bright yellowish green luminescent halos around the U-rich thorianite crystals. (C) Rutile needles ( silks, on the left) and polycrystalline böhmite (on the right). The aspect of these inclusions in this carat blue sapphire from Myanmar proves that the stone has not been subjected to HT treatment. (D) Heat treatment in sapphire causes the diffusion of Ti from rutile inclusions into the corundum, enhancing its blue color and leaving in some cases ghosts of rutile silks, as in this treated 6.46-carat sapphire. (E) This image is typical of e-diffusion treatment applied to a sapphire from Songea (Tanzania); yellowish and orangey zones of color were induced by the treatment. The observation of blue diffusion halos ( frog eggs ) around the stubby rutile inclusions typical of Songea is indicative of both heat treatment and the geographic origin. This exemplifies how, in some cases, classical gemology can detect e-diffusion treatment, which otherwise requires microdestructive LA ICP MS or LIS analysis of e when it leaves no typical traces. Field of view ca. 2 mm. Mi c r o g r a p h s A,,C,D: F. Notari; E: E. Fritsch Using X-rays for Chemical Analysis Major and trace elements in gemstones can be analyzed by secondary X-ray emission spectroscopy (see Rossman 2009 this issue). Energy dispersive X-ray fluorescence (ED-XRF) is the most common technique, as it requires no sample preparation. Analytical scanning electron microscopy is also commonly employed, especially the more recent variable pressure instruments that allow observations and qualitative analyses without the need to coat the samples with a conductive layer. The trace element content of gemstones can be used to discriminate natural from synthetic stones, different origins of natural stones, or the method of synthesis. This is especially useful for rubies with no visible inclusions or color heterogeneities (Muhlmeister et al. 1998). Similarly to spectroscopic methods, comparison of trace element compositions must be done cautiously; to be meaningful, the technique should be applied only to stones with similar color and the analyses should be performed away from microscopic inclusions. El e m e n t s 167

6 Microdestructive Techniques Even if the rule is that gem analyses must be nondestructive, it might be necessary in some cases to resort to microdestructive techniques, some of which are becoming increasingly popular in gemology laboratories. In these techniques, a tiny quantity (typically 10 to 50 microns in diameter) of the gemstone is sampled, which is invisible to the naked eye. The method is therefore considered to be acceptable, especially if the analysis is made on the girdle of the stone. The main microdestructive techniques in gemology are laser-induced plasma (or breakdown) spectroscopy (LIPS or LIS) and laser ablation inductively coupled plasma mass spectrometry (LA ICP MS). oth techniques allow the analysis of trace elements with detection limits down to the ppm level (or lower) and require no preparation of the samples. Moreover, both techniques allow the detection of light elements, for example, e in e-diffused sapphires, which is currently impossible with other available techniques. Secondary ion mass spectrometry (SIMS, commonly referred to as ion probe ) also offers unique possibilities for the analysis of trace elements and isotopic compositions and is even less destructive than laser-based techniques. Giuliani and coworkers have traced the origin of emeralds and corundum, based on their oxygen isotope composition as determined with SIMS (e.g. Giuliani et al. 2000). SIMS, however, requires specific sample preparation and is currently so complex and expensive a technique that it cannot be used routinely for gemstone analysis. AND THE GAME GOES ON With the discovery of new gem deposits, such as the recently discovered Winza ruby deposit in Tanzania, previously unknown inclusion parageneses come to light, as well as new trace element compositions and spectral features. As new methods of synthesis and treatment are devised, gemology has to adapt by developing new methods of identification and by using new instruments. At the moment, major laboratories are refining techniques to identify gem-quality synthetic diamonds grown by the chemical vapor deposition (CVD) technique (Hemley et al. 2005). Hexagonal moissanite (SiC) can be easily distinguished from diamond on the basis of its birefringence (Fig. 3), but silicon carbide is a polytypic structure, and although manufacturers have failed so far to master the synthesis of gem-quality SiC with polytypes different from the usual 6H one, it is to be expected that cubic (polytype 3C) moissanite will eventually appear on the market, depriving gemologists of a simple identification criterion. REFERENCES oillat P-Y, Notari F, Grobon C (2001) Luminescence sous excitation visible des diamants noirs irradiés: Les luminescences d arêtes. 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