Tanzanite is the blue to violet transparent variety of the mineral zoisite and is named after the locality in which it was first discovered – Tanzania. Zoisite can also be found in pinks, yellows, greens and browns with many of these other colours turning blue to violet when heated to 300°C – 400°C. In fact heat treatment is now so wide spread that Tanzanite is assumed to be heated and this has little impact on price.
Tanzanite possesses remarkably strong pleochroism, an optical phenomenon in which a mineral displays different colours when observed at different angles. In fact tanzanite is trichroic, meaning that it displays three different colours – violet, green and blue in untreated stones. After heating, tanzanite becomes dichroic meaning that only two colours can be visible, blue and violet.
There is no universally accepted method of grading coloured gemstones and so TanzaniteOne, a major player in the tanzanite market, has introduced its own. The system divides tanzanite colours into a range of hues between blue violet and violet blue. The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning that it is normally eye flawless. Gems with eye visible inclusions will be traded at large discounts.
Other gemstones that are sometimes used to simulate tanzanite are iolite, blue glass, blue YAG, purplish blue synthetic corundum and synthetic blue forsterite. A qualified gemmologist should have little difficulty in distinguishing these from the genuine article.
Because tanzanite is quite a soft mineral (6.5 on Mohs scale of hardness) it is often seen mounted in earrings and necklaces. The largest crystal ever found weighed 16,839 carats (3.4 kg) and measured 22 cm × 8 cm × 7 cm. The Tanzanian government generates approximately US$20 million annually from the mining of tanzanite with most gems being sold on the US market. Sales total approximately US$500 million annually.
If you have any tanzanite jewellery that you would like to have valued then please contact Simon Rufus, Grand Auctions' qualified gemmologist, in Folkestone, Kent.