Probably 99.99 percent of tanzanite (blue zoisite) is heated to get the gemstone’s characteristic saturated purple-blue color. In fact, most dealers will tell you it’s not tanzanite until it’s heated to create that color. But though rare, there are natural purple-blue tanzanites.
To acquire natural-color tanzanite, you must buy rough, says Dana Schorr, tanzanite specialist and owner of Schorr Marketing and Sales in Santa Barbara, Calif. “You buy parcels of rough, because people generally don’t treat the rough.” Heating rough can extend fractures or cause inclusions to explode, both of which result in more waste and smaller stones. “You want to trim off all of the cracks and inclusions that may cause problems later,” Schorr advises.
If you do find natural-color tanzanite, the stones typically will be large. “Especially in larger stones, there’s a lot of rough that you can cut into natural,” Schorr says. Larger rough will hold color better than smaller goods after cutting, which holds true for any gem, heated or not. But is it smart to cut a large piece of tanzanite rough without heating it? “If I heated it, it would be a more pure blue, and therefore easier to sell,” Schorr says. “The market just isn’t demanding natural tanzanite.”
And that’s the ultimate reason we don’t see much natural-color tanzanite. Rarity is not the primary issue. “So if I were buying kilos of rough, unless I could develop a market for only natural tanzanite, well …” Schorr doesn’t say it, but the implication is clear: He wouldn’t bother if nobody wanted it. “Now there isn’t a market for it. Everybody accepts tanzanite as being heated.”
In the past year, “rough prices have gone up significantly,” says Schorr. “It’s now leveled off over the past few months.” Prices at this time last year were in the $275/ct. to $325/ct. range for top color. Today’s asking prices are up to $450/ct. to $550/ct. Schorr has even heard of some of the larger cutters asking $500/ct. for 3-, 4-, and 5-ct. stones in top color.
Now Afgem, the professional mining operation that has contracted for block C of the Merelani tanzanite mines, may be producing more out of its own mines. And its goods in South Africa are apparently wholesaling at $550/ct. This appears to give the company control and allows it to push up the price.
But with rough prices going up, not everything is rosy. “Demand is down, but supply is way down,” says Schorr.
Afgem is a big buyer in the field and also has its own production. If a tremendous supply were available—as in the past—Afgem wouldn’t be able to buy up enough goods to have any effect on the markets.
The mines are difficult to work, production is limited, and Merelani is really the only commercial site for tanzanite. According to estimates, the site is viable through 300 to 500 meters deep, and Afgem may be the only company in a position to dig down that far.
Bottom line: Schorr and others could buy rough and have it cut and guarantee that it is natural, but there’s no market for it. “It’s all about the color,” says Schorr. “People want that really saturated blue—without the green, without the brown in it.” And unless you heat-treat it, chances are you won’t get that super-saturated blue without the yellow.
Heat treatment can be performed on rough, noted one prominent gem identification laboratory director, and the heat goes only as high as 450 degrees Celsius. Most are enhanced at about 400 degrees Celsius, so there shouldn’t be too much damage even when heating the rough.
What this means is that anyone trying to identify tanzanite had better be proficient at it. There are two methods, explains Franck Notari of Gem Tech Laboratory, Geneva, an expert in identification of heated tanzanite.
Notari uses UV-Vis spectroscopy, which employs a spectroscope that examines peaks in the ultraviolet and visible light spectrum. Zoisite is naturally opaque to ultraviolet rays, but heat treatment creates a small transmission window. For a well-equipped laboratory, testing for UV transmission is an easy and 100 percent positive test. However, most small laboratories are either not equipped for or are unfamiliar with this type of identification.
For the retail jeweler, there is a relatively easy test, but it’s not completely accurate, warns Notari. It involves using a dichroscope to examine the three pleochroic colors found in tanzanite.
The three colors are purple, blue, and a greenish-brownish-yellow. The purple is found looking down the alpha direction of the crystal axis, blue is found looking in the beta direction, and yellow is found looking in the gamma direction axis. If there is no yellow, then the stone has been heated. Removing the yellow component adds an extra blue direction. Seeing a strong yellow pleochroic color proves the stone is unheated.
The test is 90 percent accurate; seeing the yellow color depends on the quality of the dichroscope. Using a calcite scope, you will see the greenish-brownish-yellow in unheated zoisite. Using a Polaroid dichroscope, you will more than likely see brown.