A cobalt-based coating intended to improve the colour of tanzanite is the latest treatment to cause a stir.
The American Gem Trade Association Gemological Testing Center (AGTA GTC) and American Gemological Laboratories (AGL) has issued a warning following tests conducted on several samples that showed colour loss after light repolishing.
This is the first time such a coating has been identified. The samples were sent for testing by a California gemstone supplier.
“Until now, we had not identified a coating on tanzanite to improve its colour,” said AGTA GTC director Lore Kiefert.
The groups say examination with a microscope and in immersion provided clear indications of the coating. Additionally, abrasions along facet junctions and at the culet where the coating had worn off also pointed to a coating. Advanced analytical testing using X-ray fluorescence spectroscopy later verified the material was partly composed of cobalt.
Christopher P. Smith, AGL vice-president and chief gemmologist, said the tanzanite samples comprised smaller calibrated stones, which are rarely sent to laboratories for testing and can easily avoid detection.
“This is just a further reminder that each and every gemstone should be fully examined to determine whether or not it has been treated. Today, it is not uncommon to see stones that have been treated using multiple or compound techniques to achieve a particular result.”
