Lab Notes Gems & Gemology, Spring 2014, Vol. 50, No. 1

Titanium-Coated Tanzanite

Titanium-coated tanzanite
Figure 1. These five tanzanite samples (0.39–0.82 ct) proved to be color-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 gemological testing revealed a refractive index of 1.689–1.700 for all five samples. When observed using polarized light, each displayed a medium pleochroism. Specific gravity, measured using the hydrostatic method, was 3.37. These properties were consistent with tanzanite.

Tanzanite pavilion facet
Figure 2. This upper portion of a pavilion facet, examined in reflected light, shows a much higher luster than the lower, uncoated portion. Photo by Nathan Renfro; field of view 1.22 mm.
Chipped and worn coating
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 color of the tanzanite underneath. Photo by Nathan Renfro; field of view 2.90 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 luster than the crown. Some individual facets even showed a variation in luster due to an unevenly distributed color 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 also consistent with a color coating (figure 3).

Because tanzanite with a cobalt coating has previously been reported (S.F. McClure and A.H. Shen, “Coated tanzanite,” Summer 2008 G&G, pp. 142–147), 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 color-enhancing coatings on tanzanite are occasionally seen at GIA’s laboratory, this is the first time we have examined tanzanite that has been color coated with titanium.

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