Emeralds Partially Coated with Amorphous Carbon
by Gagan Choudhary
(This article was first appeared in The Journal of Gemmology, 34(3), 2014, pp. 242-246)
Recently, nine faceted emeralds submitted to the Gem Testing Laboratory, Jaipur, India, were identified as coated owing to the metallic to sub-metallic reflections on their pavilion facets. The coated surfaces did not show any diagnostic features with routine EDXRF and Raman spectroscopy. However, some samples had concentrations of the coating substance in surface cavities, and Raman analysis of those areas revealed the presence of amorphous carbon. Specifically, the presence of a broad absorption feature at ~1550 cm-1 with a shoulder at ~1360 cm-1 identified the coating substance as an ‘a-C’ type film. Microscopic observation showed that the coating was damaged and removed from several areas, suggesting its instability to normal wear and tear.
The coating of gems is one of the oldest forms of enhancement, and is done to improve the appearance and/or durability of the stone, and thereby its value. The use of traditional coating materials such as paint, ink, plastic or coloured polymers still continues today (e.g. Choudhary, 2011; 2013), but they have been largely replaced by more sophisticated coatings. With advances in technology, improved coating methods have been developed to provide a wider range of colors and optical effects with better durability. During the past several years, a variety of gem materials such as diamond, topaz, quartz, beryl and cubic zirconia have been coated with metals (gold, silver, etc.), oxides (of aluminium, silicon, zirconium, etc.), or fluorides (of calcium, magnesium, etc.), and many of these have been described in the literature (e.g. McClure and Smith, 2000; Evans et al., 2005; Shen et al., 2007; Schmetzer, 2008). In addition, treaters have claimed using other types of films—such as diamond-like carbon (DLC) and nanocrystalline synthetic diamond—to reportedly modify the colour, appearance and/or durability of diamonds and coloured gemstones (e.g. McClure el al., 2010; Shigley et al., 2012). Serenity Technologies (www.serenitytechnology.com), one of the companies that performs such treatments, claims to use a nanocrystalline diamond coating process to improve surface wear resistance of various ‘soft’ gem materials, including emerald, apatite, tanzanite and others. A Japanese company, Apple Green Diamond Inc., is marketing various synthetic coloured gemstones (as well as cubic zirconia) that also reportedly have nanocrystalline diamond coatings (http://diamondlite.co.jp).
Although coloured gemstones supposedly coated with DLC have been available in the trade for years (Koivula and Kammerling, 1991), no detailed documentation of these materials is available to the author’s knowledge. Recently, the author examined nine faceted emeralds (Figure 1) at the Gem Testing Laboratory, Jaipur, that were identified as coated with an amorphous carbon film. According to the client, these stones made their way to Jaipur from Hong Kong. This article provides a brief characterization of these coated emeralds.
Figure 1: These nine faceted emerald samples (3.54–33.64 ct) proved to be coated with an ultra-thin carbon-based film on their pavilion facets.
Background on Carbon Film Coatings
Broadly, carbon films have been divided into three types: amorphous carbon (a-C), nanocrystalline and microcrystalline. In materials science, diamond-like carbon is defined as amorphous carbon containing an unstructured mixture of sp2 (as in graphite) and sp3 (as in diamond) bonds, resulting in variable hardness, chemical inertness, transparency, colour, etc. (e.g. Robertson, 2002; Filik, 2005). The higher the sp3 content, the greater the hardness and durability of the material. DLC films exist in various sub-forms, depending on their structure and method of production; a few examples are ‘a-C:H’, ‘ta-C’, ‘ta-C:H’ and ‘polymeric a-C:H’ (e.g. Chu and Li, 2006). However, in the gem industry, treaters frequently claim they use a ‘diamond-like-coating’ rather than amorphous carbon. Members of the gem trade commonly associate such coatings with diamond and simply call the gems ‘diamond coated’, but these amorphous carbon or diamond-like carbon films do not have the hardness or durability associated with diamond. Nano-crystalline and micro-crystalline carbon films have quite different structures and properties than amorphous carbon films, and can be associated with diamond (again, e.g. Chu and Li, 2006).
Samples and Methods
Nine emeralds, weighing 3.54–33.64 ct (figure 1), were submitted to the Gem Testing Laboratory, Jaipur, for routine identification at without any prior information. Standard gemmological testing was performed to establish their identity. Qualitative energy-dispersive X-ray fluorescence (EDXRF) chemical analyses of all samples were conducted using a PANalytical Minipal 2 instrument under two different conditions: Elements with a low atomic number (e.g., Si) were measured with a tube voltage of 4 kV and current of 0.850 mA, while transition and heavier elements were measured at 15 kV and 0.016 mA. Raman spectra in the region 2000–200 cm-1 were obtained using an Airix Corporation – TechnoS Instruments STR 300 confocal microspectrometer with 532 nm laser excitation, an exposure time of 10 seconds per scan, and 10 scans per spot.
Results and Discussion
When viewed face up, all samples displayed a similar green colour with a slight yellowish tint and moderate saturation (again, see Figure 1) that is typically associated with emeralds. The samples appeared transparent with minor to significant inclusions visible to the unaided eye, and had a vitreous lustre. These initial observations suggested the samples were natural emeralds. However, when their pavilion side was viewed, a golden metallic to sub-metallic lustre was apparent (Figure 2), which raised the suspicion that a surface-related treatment, such as coating, was present on those facets.
All the measured gemmological properties were consistent with emerald. Although it would not have been surprising to find some difference in refractive indices between the crown and pavilion surfaces due to the coating on the pavilion, identical RI values were obtained with similar sharpness of the shadow edges on the refractometer scale.
Viewed with the gemmological microscope, all the emeralds displayed typical liquid films, ‘fingerprints’ and prominent jagged three-phase inclusions along with growth and colour zoning, confirming them as natural and of probable Colombian origin (e.g. Gübelin and Koivula, 1997). In addition, all the emeralds had been clarity enhanced using a colourless resin, in amounts ranging from minor to significant. The presence of resin was suggested by golden/blue colour flashes along the fractures, and this was later confirmed by infrared spectroscopy (cf. Kiefert et al., 1999).
Viewed with reflected light, even at higher magnification, the crown side of the emeralds did not display any features related to surface coating, while the pavilions showed a golden metallic to sub-metallic lustre with some chipped areas (Figures 2 and 3) that displayed the vitreous lustre of the underlying emerald. In addition, some surface-reaching fractures stood out as highly contrasting dark lines against the bright surrounding facet. When the samples were observed in diffused transmitted light, the coating substance appeared brown (Figure 4), particularly where it was more visible around the chipped off areas, enabling a comparison between the coated surface and the underlying emerald. Some of the cavities and/or surface-reaching tubules appeared darker than the rest of the coated surfaces, suggesting a thicker concentration of the coating substance in those areas (Figure 5). Under oblique fibre-optic lighting some of the facets also displayed a strong blue iridescence along with fine lines that appeared to be polishing marks (Figure 5). The coating substance apparently scattered the white light from the polishing lines/grooves, resulting in the blue iridescence.
Although the metallic to sub-metallic lustre of the coating suggested the presence of some metallic oxide, EDXRF spectroscopy revealed only those elements associated with emerald: impurities of Ca, V, Cr and Fe were present along with Al and Si. Traces of Ti, which provide one of the key identifying features of the coating process associated with Diamantine (Shigley et al., 2012), were not detected in these samples.
Raman spectroscopy of a typical coated surface did not produce any features besides those associated with the underlying emerald, but this was not surprising considering the extremely thin nature of some coating materials. However, Raman analysis of surface-breaking cavities and growth tubes that displayed concentrations of the coating substance showed a broad absorption feature at ~1550 ± 5 cm-1 with a shoulder at ~1360 ± 5 cm-1 (Figure 6). The feature at ~1550 cm-1 is designated as the 'G' peak for graphite, while that at ~1360 cm-1 is a 'D' (disorder) peak (e.g. Chu and Li, 2006; Mednikarov et al., 2005); both of these features are due to sp2 bonding of carbon atoms. Further, the Raman spectral pattern observed for this coating substance was consistent with that reported for a-C films (i.e. softer carbon films formed without hydrogen, usually at low energy or higher temperature; Chu and Li, 2006). The characteristic strong peak of diamond at 1332 cm-1 was completely missing from the coating substance.
Figure 6: Raman analysis of surface-breaking growth tubes (see Figure 5) containing thicker concentrations of the coating substance produced a broad absorption feature at ~1550 cm-1 with a shoulder at ~1360 cm-1. This spectral pattern is consistent with amorphous carbon films and is due to sp2 bonding of the carbon atoms.
The identification of these emeralds as coated was quite straightforward because of the presence of metallic to sub-metallic surface reflections on the pavilion facets. However, determining the nature of the coating substance was more challenging, as reported previously for ultrathin carbon-based coatings (e.g. McClure et al., 2010; Shigley et al., 2012). Fortunately, four samples had concentrations of the coating substance in surface features, and Raman spectroscopy of these areas identified the coating as amorphous carbon. Although technically such coatings may be referred to as ‘diamond-like carbon’, in the gem trade this terminology is often falsely equated to diamond (i.e., ‘diamond coated’). However, these coatings do not possess the hardness and durability associated with diamond, and therefore must not be called ‘diamond coated’.
Gemstones are typically coated to improve their appearance and/or durability, but in this case a lot has been left for assumption regarding the improvement of both these factors. These emeralds were coated only on their pavilion facets, which implies that the treatment was intended to improve their brilliance. In a stone like emerald, which is rarely inclusion-free, such a coating will hardly make an impact. Further, the coating on these emeralds had been chipped off of several areas, suggesting its instability to normal wear and tear.
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AcknowledgementsThe author is grateful to Dr Tarun Sharda of Jaipur for helpful discussions on diamond films, to the anonymous reviewers for their help with nomenclature of carbon-based coating materials and to Sandeep Vijay of the Gem Testing Laboratory for recording Raman data.
All photographs and photomicrographs by Gagan Choudhary