Natural diamonds with a strongly saturated pink hue are rare, but a number of treatment techniques have been developed to induce or enhance a pink hue in natural and synthetic diamonds. These methods include irradiation and annealing, high pressure/high temperature annealing, and coating.
Recently, three intensely colored pink diamonds (0.70–1.05 ct.) were submitted to GIA’s East Coast laboratory for identification. The colors were pink, orangy pink, and purple-pink (figure 1). When observed with magnification and regular diffused light, the pink coloration seemed evenly distributed, particularly when the diamonds were viewed face up. However, when the stones were immersed in methylene iodide, we saw distinct pink color concentrations at the facet junctions. With reflected diffused light, an iridescent film-like coating – seen as interference colors on the surface – was easily visible on the pavilion facets of all three stones (figure 2). In addition, we noted random colorless spots and lines on all the coated facets. Careful examination of all three diamonds confirmed that the coating was restricted to the pavilion; no coating was observed on the table or crown facets.
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Figure 1. The intense pink color of these three diamonds (0.70–1.05 ct.) proved to be the result of a surface coating. Photo by Elizabeth Schrader.
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Infrared absorption spectroscopy showed that the emerald-cut stone (far left in figure 1) was type IIa, while the other two stones were type IaAB with low to high concentrations of nitrogen. In the UV-visible spectra of all three stones, a broad absorption band was centered at ~530 nm, similar to the ~550 nm absorption band seen in most naturally colored pink diamonds. However, the peak at ~390 nm, which always appears with the 550 nm band in natural pink diamonds, was not present. These spectroscopic features supported the visual observation that the pink color resulted from a coating treatment rather than lattice defects, as would be the case with natural pink diamonds. As a cautionary note, it is possible to get a good pink color appearance by coating diamonds that have a slight brown hue; some brown diamonds have broad absorption bands at ~550 and 390 nm, in which case the spectra of the coated stones will appear more “natural.”
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Figure 2. An iridescent film was observed on the pavilion facets of all three stones when viewed with reflected diffused light. In addition, colorless spots and lines were seen on all the coated facets. Magnified 60×; photomicrograph by Wuyi Wang.
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Identification of diamonds treated in this fashion is usually straightforward. Nevertheless, coated diamonds have occasionally been submitted to the laboratory for identification or grading. Although we did not try to identify the nature of the coatings on these three diamonds, D. J. F. Evans et al. (“Coated pink diamond: A cautionary tale,” Spring 2005 Gems & Gemology, pp. 36–41) performed elemental analysis on the coating of a treated pink diamond they studied. They suggested it might be calcium fluoride doped with gold. Since sophisticated film-coating techniques have long been used in the optical industry, we suspect that applying such a thin film over the pavilion facets of loose diamonds can be easily achieved in large quantities. We do know that a number of coated pink diamonds have entered the marketplace.
As we have noted previously, it is GIA policy not to issue grading reports for diamonds treated with foreign materials (e.g., coating, fracture filling), since these treatments are not permanent. Accordingly, these three stones were returned to the client with an identification report only.
Wuyi Wang, Thomas Gelb, and Surjit Dillon
GIA Laboratory, New York
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