The American Gem Trade Association Gemological Testing Center (AGTA-GTC) and American Gemological Laboratories (AGL) received a number of faceted tanzanite samples recently in New York that were determined to have been coated.
Evan Caplan of Omi Gems Inc. sent samples to several labs after a light repolishing of a few stones resulted in a noticeable loss of color, according to AGTA-GTC.
“Until now, we had not identified a coating on tanzanite to improve its color,” said Dr. Lore Kiefert, director of the AGTA-GTC.
Christopher Smith, AGL vice president and chief gemologist, said, “Although the coating is not immediately obvious, careful examination with a microscope and in immersion provided clear indications of the coating in most instances.
“This was evident by abrasions along facet junctions and at the culet where the coating had worn off, as well as a subtle iridescence when viewing the surface with reflected light,” Smith said.
Advanced analytical testing identified that the coating contained cobalt. “The most reliable means to substantiate the presence of the coating is the use of X-ray fluorescence spectroscopy,” Kiefert said. “The coating is colored by cobalt, which is readily detected using this analytical technique.”
Although the gemstone industry has become very familiar with the practice of heating zoisite to achieve the best violet to blue color in tanzanite, these stones represent the first time either lab has identified a color-enhancing coating on tanzanite, the labs reported in a joint statement.
The majority of the tanzanite sample was comprised of smaller calibrated stones. Fine color tanzanite in this size range is rarely sent to a lab and therefore would avoid detection unless closely scrutinized. “This is just another reminder that each and every gemstone should be fully examined to determine whether or not it has been treated,” Smith said. “Today, it is not uncommon to see stones that have been treated using multiple or compound techniques to achieve a particular result.”
Kiefert and Smith emphasized that any treatment used to modify the color of a gem should be disclosed.
"Coatings in particular are not considered permanent and in the U.S. and are required by FTC guidelines to be properly disclosed at the point of sale,” they reported in a statement.
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