While grading and testing tanzanites, the American Gem Trade Association Gemological Testing Center (AGTA-GTC) and the American Gemological Laboratories (AGL), discovered the stones to be coated with a colour enhancement coating. The samples came from Evan Caplan of Omi Gems, Inc. who had previously sent the stones to various labs, as a light re-polishing on then had caused a noticeable loss of colour. %% This is the first such incident which the two labs had come across, as tanzanites were not known to be coated previously. “Until now, we had not identified a coating on tanzanite to improve its colour.” indicated Dr. Lore Kiefert, Director of the AGTA-GTC. The coating was discovered during analytical testing, to contain cobalt. “The most reliable means to substantiate the presence of the coating is the use of X-ray fluorescence spectroscopy.” Kiefert explained. %% The diamond and gem industry is familiar with the practice of heating zoisite to achieve an optimum violet to blue colour in tanzanite, but it is for the first time that colour enhancement coating was discovered. “Although the coating is not immediately obvious, careful examination with a microscope and in immersion provided clear indications of the coating in most instances.” stated Christopher P. Smith, Vice President and Chief Gemologist of AGL “This was evidenced 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 further added. %% Most of the samples sent to the labs were of smaller calibrated stones, which are rarely sent to a lab, resulting in a lack of detection. “This is just another reminder that each and every gemstone should be fully examined to determine whether or not it has been treated.” Smith indicated “Today, it is not uncommon to see stones that have been treated using multiple or compound techniques to achieve a particular result.” Coatings are not of permanent nature. Retailers and manufacturers in the U.S. are required to disclose if the stone is treated at the point of sale according to FTC guidelines.