Tanzanite is an enormously popular gem, and the discovery of stones coated with a color-enhancing substance has generated controversy within the trade.
In the Summer 2008 issue of Gems & Gemology, GIA researchers Shane McClure and Dr. Andy Shen offered an extensive analysis of these treated stones and the means to identify them.
The authors examined 23 tanzanites, two of them large stones over 3 carats and the rest between 0.27 and 0.62 carats (see, e.g., figure 1), which were coated by an unknown firm and put on the market undisclosed this spring.
Los Angeles dealer Omi Gems brought these tanzanites to the lab’s attention because they suspected some form of treatment. Even 4-5 mm stones were strongly colored. All proved to be coated, but it was very difficult to detect the coating on the two largest tanzanites (3.01 and 3.21 ct). Photo by C.D. Mengason. © 2008 Gemological Institute of America (GIA).
All the tanzanites were treated by the same process, a coating that contained a significant amount of cobalt. The same element was used in a coating examined two years earlier that was applied to the pavilion of a tanzanite, purportedly to enhance its durability.
The coatings were difficult to detect on the two larger stones. They did not show clues typically associated with this treatment, such as faint iridescence in reflected light or gaps and worn-off areas in the surface material when examined with10× magnification. Nor had the coating obscured the polishing lines, which are often covered by such a treatment. In addition, because the tanzanite had some intrinsic color before being coated, the stones still exhibited some of the pleochroism (the appearance of multiple colors when a stone is viewed from different directions) that is characteristic of tanzanite.
Only with higher (60×) magnification did the researchers find telltale signs of coating on the larger stones, in the form of tiny holes in the surface and whitish marks that resembled dirt but couldn’t be wiped off, as well as orange iridescent lines that crossed facet junctions (figure 2).
Numerous whitish marks that resembled dirt (but could not be wiped off) were visible on the most of coated tanzanites with fiber-optic illumination, which also revealed orange iridescent lines crossing facet junctions on some stones. Photomicrographs by S.F. McClure. © 2008 Gemological Institute of America (GIA).
The smaller goods proved easier to identify. For the most part, all had an intense violet-blue color that is rare in such small tanzanites. Magnification revealed areas along some facet junctions where the coating was worn off, probably because the stones had been stored in group parcels, causing abrasion (figure 3). These smaller stones also showed weak iridescence in reflected light (figure 4).
Abrasion of the coating at facet junctions was plainly visible when the smaller tanzanites (here, about 5 mm) were immersed in water over diffused transmitted light. Photomicrographs by S.F. McClure. © 2008 Gemological Institute of America (GIA)
Scratch-testing the coating with an ordinary straight pin caused no damage, but a hardened chrome-steel knife blade did cut through the material.
All the smaller tanzanites examined for this study showed a weak iridescence in reflected lint, as well as areas along facet junctions or the culet where the coating had worn off. Photomicrographs by S.F. McClure. © 2008 Gemological Institute of America (GIA).
Advanced chemical testing with laser ablation–inductively coupled plasma–mass spectrometry revealed some elements, including cobalt, that were present on the surface but not deeper inside the stone.
Anyone handling tanzanite should check carefully for this new coating, especially on larger stones that are kept in individual gem papers and not subjected to the scratches and abrasions that render the coatings easier to recognize. Note, too, that the coated tanzanites examined at GIA were sold into the market without disclosure (and supplied to GIA by a dealer who had inadvertently purchased them)--so there are likely to be more such goods in circulation. Repolishing of one of the stones by that dealer, Omi Gems of Los Angeles, revealed that the original tanzanite was significantly lighter than the coated material (figure 5).
These two 5 mm stones were originally the same color. After repolishing, the one on the right was significantly lighter. Photo by C.D. Mengason. © 2008 Gemological Institute of America (GIA)
. © Gemological Institute of America.