The coating of gemstones to improve their color has been known for thousands of years, although the typical nonpermanence of the treatment has made it impractical at best and fraudulent at worst. Last year, GIA researchers reported on the emergence of a new generation of coatings—created using technology from other industries—that are much more durable than those studied previously (A. H. Shen et al., "Serenity Coated Colored Diamonds: Detection and Durability," Spring 2007 Gems & Gemology, pp. 16–34). That report described colored coatings on diamonds by Serenity Technologies, although we are aware of other companies that are using a similar treatment on diamonds.

During our investigation of these new coating operations in 2006, we also examined some tanzanite that purportedly had been coated to enhance its durability. Based on the samples we examined at that time, the durability problems and ineffectiveness of the coating did not distinguish this technique from most of the coating techniques reported in the past.

In April of this year, however, a Los Angeles colored stone dealer contacted us about a parcel of tanzanite his company had just received from New York. Evan Caplan of Omi Gems said the color did not look right, and when the company had one stone repolished it became significantly lighter. This sounded like a coating, so we asked Mr. Caplan if we could examine some of the stones. Initially, Omni Gems loaned GIA two 3-plus ct emerald cuts (figure 1) and four smaller (4–5 millimeter) rounds. Examination of the emerald cuts at about 10× magnification did not reveal any obvious coating characteristics. Coatings are usually visible in reflected light or by looking through the table of the stone toward the pavilion in transmitted light. The features we typically look for—iridescence in transmitted light, or gaps or worn-off areas in the coating—were not immediately evident. In fact, polishing lines were plainly visible on most of the facets.

On closer inspection at higher magnification, however, we observed a number of clues. With reflected light, we saw tiny holes in the coating. These holes seemed relatively sharp-edged, and the coating appeared thicker than we typically see in such material. Fiber-optic illumination revealed whitish marks on the surface that looked like dirt but did not wipe off. We also saw several orangy iridescent lines that crossed facet junctions, which were similar in appearance to lines that might be left by a liquid drying on the surface. In addition, there were tiny bright pink to orange flashes of light that turned out to be related to minute areas of damage on the coating.

At our request, Omi Gems sent us 18 more samples, 4–5 millimeter rounds. Each had a depth of color that would be very unusual for tanzanites this small. In addition, when examined with magnification and reflected light, all the stones showed some characteristics that would be associated with a coating: a pale iridescence on the surface, as well as wear at facet junctions or on the culet that is typical of the "paper wear" often present on gems that are stored together in stone papers. The worn facet junctions were readily apparent with immersion in water, since they appeared lighter in color than the rest of the stone. All the features observed in the smaller stones are consistent with those reported by the American Gem Trade Association Gemological Testing Center and American Gemological Laboratories in their joint May 23 press release on this new treatment.

Energy-dispersive X-ray fluorescence analysis of several samples showed cobalt as a major constituent of the coatings. This chemistry is consistent with the coatings on tanzanite we examined in 2006. In all 20 samples tested, significant amounts of cobalt were found only on the pavilion facets, not on the crown.

To summarize, it is important to emphasize that these coated tanzanites were sold on the market in New York undisclosed. The smaller stones were immediately suspect because it is very uncommon for tanzanites in these sizes to show such saturated color. All exhibited abrasion on facet junctions or the culet, probably from paper wear, which makes the coating easier to detect. Larger stones that are stored individually, however, may not show these features, and some do not show iridescence in reflected light. Detecting these stones requires careful observation.

More details on the identification of this coated tanzanite are provided in an article that has been submitted for possible inclusion in a future issue of Gems & Gemology.

Shane F. McClure
GIA Laboratory, Carlsbad, California