A year ago, there wasn’t much new to be said about tanzanite, the oven-blued zoisite found only in its namesake country of Tanzania. Known to be benignly heated from brown to blue since its discovery in the mid-1960s, tanzanite was one of the gem world’s safer, most worry-free precious stones.
Then, last summer, this gem suffered its first major gemological scandal. Dealers started seeing lots of melee and calibrated goods with exceptional color rarely seen in smaller sizes—as well as larger single stones with remarkably uniform color. Working with goods submitted by dealers, American Gemological Laboratories and AGTA’s Gem Testing Center jointly discovered that some stones were being coated with cobalt to give them their stellar color. Once detected, the labs quickly devised a regimen of easy tests to ferret out suspect goods.
Although these labs and others were able to get the information into the trade quickly, random or batch testing of tanzanite did not fully alleviate the problem, especially with smaller, calibrated goods. As a result, Chris Smith at AGL developed a chemical bath which completely eliminated the coating without harming the underlying tanzanite. “I felt that the industry needed a more practical method for dealing with the stock already in the possession of wholesalers, manufacturers and retailers,” he recalls. “Many in the trade were concerned that they might already have coated tanzanite in their stocks and they did not want to inadvertently sell coated material.” The tanzanite bath allowed stones to submersed and come out with a clean bill of health.
In conversation with the Tanzanite Foundation and industry organizations, it was decided to withhold the cobalt remover’s formula. That forced Smith to occasionally strip coatings as a favor for customers of AGL (which, by the way, Smith has just announced he is planning to acquire from its former owner since 2006, Collector’s Universe). “I developed the bath as a way to make sure tanzanites were coating-free and thus re-establish confidence in one of our industry’s most popular colored stones,” Smith says. “Otherwise, trades people would have had to spend considerable time and money batch or randomly testing material.”
Blued By Nature
After spending months studying tanzanite, Smith decided last summer to visit the Merelani hills which are the only known deposit for this gem. “It was just a courtesy and curiosity call,” he says. “I wanted to learn more about the mining and production of tanzanite. And the best way to do so was by first-hand observation.”
Because every tanzanite that is mined is sent for heating, it is assumed that all of these zoisites need oven time to be baked to this gem’s distinctive violet-blue. But during a visit to TanzaniteOne, the De Beers-like mining and marketing organization for most of the world’s tanzanite, Smith found that a significant number of stones had already been blued in the ground—hence treating wasn’t necessary in all cases.
“I was quite surprised to see a number of stones were coming from the ground with a natural-blue color. So I started a project to determine a means of identifying natural-color tanzanite. As part of this study I found there were three kinds of stones being mined,” he explains. “The first and by far the most predominant type are stones with a beer bottle-brown that need heating to be converted to blue; the second type have been partially annealed and display an overall blue appearance but still show brown in one direction. A third type has been fully annealed naturally and shows a complete blue coloration.”
Does this mean that there could be what Smith calls “a new niche market” for natural-color, as opposed to heated, tanzanite—similar to that for sapphire? Smith believes so, but is quick to say that applications of his findings are better left to marketers than gemologists.
Smith plans to soon publish the findings of his tanzanite study in a leading gemological journal.