|
|
These two samples of synthetic corundum (1.18 and 2.03 ct.) were sold in Minas Gerais, Brazil, as Imperial topaz. Photo © GIA 2007.
|
Topaz is a popular gem due to its attractive appearance, ready availability, and generally low price. The one exception—which is both rare and costly—is the deep yellow-orange-pink variety known as Imperial topaz. By far the most important source of this gem is the Ouro Preto area in Minas Gerais, Brazil. Unlike many other gem materials, synthetic Imperial topaz is not commercially available, and common imitations such as citrine, synthetic spinel, and glass are easily detectable.
While in Minas Gerais in August 2006, one of these contributors (MM) purchased two loupe-clean pieces of rough that were represented as Imperial topaz, which he subsequetly had cut (see figure). The two pieces had an irregular triangular shape, which the dealer had indicated would maximize the yield from the rough. The color and the vitreous-to-subadamantine luster resembled Imperial topaz.
The two pieces (1.18 and 2.03 ct.) were analyzed in the Department of Earth Science of the University of Rome "La Sapienza," and the following gemological data were obtained: color—orange-yellow-"rose"; diaphaneity—transparent; RI—no = 1.770 and ne = 1.761; birefringence—0.009; optic character—uniaxial negative; SG—4.05; fluorescence—inert to long- and short-wave UV radiation; and no inclusions were seen with a gemological microscope. These properties identified the pieces as corundum; their synthetic origin was strongly suggested by the lack of any natural-appearing inclusions (typical of flame-fusion material) and the relatively inexpensive price. The synthetic origin was confirmed by LA-ICP-MS analysis at GIA of one of the samples (donated to GIA by Mr. Macrì); the lack of Ga was characteristic of flame-fusion synthetic corundum. The instrument did find traces of Cr, Ni, Ti, and Mg, but no Be, which indicates that the color of the synthetic corundum was not influenced by Be-diffusion treatment.
Although the color and luster of this synthetic corundum are strikingly similar to Imperial topaz, the two materials can be easily separated by their standard gemological properties. Still, this imitation could present a problem for the unsuspecting buyer. Interestingly, similar-colored synthetic corundum was recently sold as spessartine in the Tanzanian market (see Winter 2006 Gems & Gemology, p. 282).
Michele Macrì and Adriana Maras
University of Rome "La Sapienza"
Andy H. Shen
GIA Laboratory, Carlsbad, California
|
