Tanzanite (Merelani)

There is one place on Earth where this stone comes from, and it is about seven kilometres long.

Tanzanite is the blue and violet variety of zoisite, a calcium aluminium hydroxyl sorosilicate, and small amounts of vanadium give it the colour. It is found only in the Simanjiro District of Manyara Region in Tanzania, in a mining area roughly 7 km long and 2 km wide near the Mererani Hills, in the Lelatema Mountains. That is the entire world supply.

Its party trick is trichroism, and it is unusually strong. Depending on which crystal axis you look down, the same stone shows blue, violet and burgundy, and the lighting changes it again: the blues come forward under fluorescent light, the violets under incandescent.

Now the part that matters if you are buying. In the rough, tanzanite is reddish brown to clear, and it takes heat treatment to strip that brownish veil and bring out the blue-violet. You should assume every blue to violet tanzanite you see has been heated. The treatment is stable, so this is disclosure rather than a defect. There are no known synthetic tanzanites, but plenty of simulants.

Ignore the block story. There are four blocks of tanzanite-producing mines in Tanzania, and the trade has long claimed that D-Block stones are of superior quality. According to the International Gem Society there is no quality difference between the blocks and no way to tell which block a stone came from, so a D-Block label is a sales line, not a specification.

The name came from a shop. Tiffany and Co. introduced the stone to the market in 1968 and named it after the country, on the view that blue-violet zoisite was not going to sell itself.

The mines are controlled ground, so buy it rather than look for it, and buy from someone who will put the heat treatment in writing.