Tanzanite is a transparent blue gemstone that was first discovered in 1962 in Tanzania, in East Africa. It is a variety of a mineral called zoisite. Initially only small pockets were discovered, but in the late 1960s a large deposit was discovered, and large-scale mining began.
Its immense and sudden popularity was almost entirely due to the fact that Tiffany and Co began a huge marketing campaign in 1968 to promote it; it was the gem of choice for cutting edge designers and sellers, and was instantly in demand. However, supply was intermittent and unreliable, and this is still true today. Prices and availability are subject to fluctuations depending on the political situation in Tanzania.
The main appeal of tanzanite is that it is highly pleochroic, that is that it will show different colours from different viewing angles. The two main colours are blue and purple. Because of the way in which the crystal grows, a cutter will typically get a larger finished stone by cutting the gem to show the purple colour. This has made predominantly blue tanzanite rarer and more valuable.
The Properties of Tanzanite
Tanzanite is graded on the mohs hardness scale at 6-7, with fair to poor toughness. (It is worth bearing in mind that this is still more durable than steel!) This means that it can be susceptible to cleaving and to thermal shock. It should not be cleaned in a steam machine, worn in a manner likely to give it a strong knock, or exposed to sudden temperature change. Most examples are mounted in necklaces or earrings, where they are less likely to receive such a knock. It is safe to clean tanzanite with warm soapy water.
Fine blue examples (“top blue”) are sometimes confused with sapphire, but there will almost always be a degree of purple visible.