Titanium-Coated Tanzanite

These five tanzanite samples (0.39–0.82 ct) proved to be colour-coated with titanium. Photo by Don Mengason.
Five faceted violetish blue stones, ranging from 0.39 to 0.82 ct (figure 1), were recently submitted to the Carlsbad laboratory for identification service. Standard gemmological testing revealed a refractive index of 1.689–1.70 for all five samples. When observed using polarised light, each displayed a medium pleochroism. Specific gravity, measured using the hydrostatic method, was 3.37. These properties were consistent with tanzanite.

Figure 2. This upper portion of a pavilion facet, examined in reflected light, shows a much higher lustre than the lower, uncoated portion. Photomicrograph by Nathan Renfro; field of view 1.22 mm.
Microscopic examination showed that the material was relatively free of inclusions, but showed areas of abrasion along pavilion facet junctions. Observed using reflected light, the pavilion of each stone showed a significantly higher lustre than the crown. Some individual facets even showed a variation in lustre due to an incomplete distribution of the colour coating (figure 2). When the samples were examined using diffuse transmitted light, the facet junctions and several chipped spots appeared much less saturated, which was consistent with a colour coating (figure 3).

Figure 3. The coating on this tanzanite’s pavilion has been worn away along the facet junctions and in some small chipped areas, revealing the less-saturated violet colour of the tanzanite underneath. Photomicrograph by Nathan Renfro; field of view 2.90 mm.
Because tanzanite with a cobalt coating has previously been reported, all tanzanites submitted to the lab are routinely checked by energy-dispersive X-ray fluorescence (EDXRF) and microscopic examination. These five stones were also checked by EDXRF, but no cobalt was detected; however, all five showed a significant signal for titanium on their pavilions. No titanium was detected on the crowns. LA-ICP-MS was also used to confirm that the coated area contained significant titanium.

While colour-enhancing coatings on tanzanite are occasionally seen at GIA’s laboratory, this is the first time we have examined tanzanite that has been colour coated with titanium.

Amy Cooper is a staff gemmologist and Nathan Renfro is lead analytical specialist of gem identification at GIA’s laboratory in Carlsbad, California.