Tanzanite
A blue-violet zoisite found in just one place on Earth — heat-treated and pleochroic.
Also known as: Blue zoisite
intermediate Merelani Hills, Tanzania
What it is
Tanzanite is the blue to violet variety of the mineral zoisite (a calcium aluminium silicate), coloured by vanadium. It is only moderately hard at 6 to 7 on the Mohs scale and has a distinct cleavage, so it needs protective settings and careful handling. Tanzanite is strongly pleochroic, showing blue, violet and burgundy depending on viewing angle, which cutters orient to favour the best blue.
Almost all tanzanite is gently heated to convert brownish rough into the desired blue-violet, a stable and universal treatment. Its defining feature is rarity of source: it is found commercially only near Mount Kilimanjaro in the Merelani Hills of Tanzania. Introduced to the market in the 1960s, it was later adopted as a December birthstone.