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Tanzanite Grading and Value
Tanzanite, a rare violet-blue form of the mineral zoisite, was first discovered in 1967 in Tanzania. East Africa remains the only source for this gems and the supply of high grade material is limited. The limited supply, combined with strong marketing, has resulting in high prices, sometimes as much as $1,000 per carat.
Tanzanite is a gorgeous gem, but its tremendous popularity must be surprising to the miners who originally discovered it. In its natural state, tanzanite is a rather unappealing brown, and the violet blue is produced through heat treatment. It is not a especially hard gem either. At 6.5 on the Mohs hardness scale and with delicate cleavage, it is less durable than common quartz. But the rich color of the best tanzanite is quite unique in the gemstone world, rivalled only by fine blue sapphire.
|Top Grade Tanzanite|
But not all tanzanite is especially valuable. The main determinants of tanzanite value, like all colored gems, are color, clarity, cut and carat weight. But in the case of tanzanite, color is so important that it outweighs almost all other considerations. Tanzanite is classified as a Type I gemstone by GIA, meaning that tanzanite specimens are normally eye clean. So clarity is rarely a problem with tanzanite. But the color range can be quite wide, since tanzanite occurs in gold, green and violet as well as blue. The saturation is also highly variable, with some stones quite pale.
So evaluating tanzanite grade is very important. Unfortunately there is no accepted standard for tanzanite grading. This may come as a surprise, since you will often hear people talk about "AAA Tanzanite". But this term has no exact definition in the gems trade. Indeed, the largest miner of tanzanite, a company called TanzaniteOne, has proposed a completely different grading scheme through their non-profit subsidiary, the Tanzanite Foundation. They grade color using terms like "exceptional", "vivid", "intense", "moderate", "light" and "pale," and grade color as "violetish blue" or "bluish violet."
Since there is no way to measure the difference, for example, between "exceptional" and "vivid," this grading scheme is unlikely to be adopted by any gemological laboratory. But one of the virtues of this grading scheme is it helps to explain why certain colors are graded higher than others. In general we can say that a tanzanite is more valuable if the color is more blue than violet, and when the color is deeper or more saturated. Though some buyers may prefer a more violet color, or a lighter tone, the deep blue tanzanites will sell at the highest prices in the market.
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