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.

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