Tanzanite

GIA and the International Gem Society state that tanzanite is mined commercially only in the Merelani Hills in the Lelatema Mountains of northern Tanzania. Zoisite forms during regional metamorphism and hydrothermal alteration of calcium-rich rocks. The deposit is worked in several mining blocks; according to the International Gem Society, material from different blocks cannot be told apart.

Tanzanite is strongly pleochroic: a fashioned stone shows blue and violet when tilted, and its colour looks bluer in cool daylight-type light and more violet under warm incandescent light. Its refractive index of about 1.69–1.70 and specific gravity of about 3.35 help separate it from look-alikes. Synthetic tanzanite is not known, but simulants exist; blue-violet stones should be assumed to be heat treated, and some are coated. The International Gem Society notes that green, yellow or pink zoisites sold as "fancy colour tanzanite" are correctly called zoisite, because tanzanite by definition is blue to violet.

Although its hardness is 6 to 7, tanzanite is brittle and has perfect cleavage, so the International Gem Society advises reserving it for occasional wear or protective settings and avoiding rough treatment. Clean it only with a soft brush, mild detergent and warm water.

According to the most common account reported by GIA, in 1967 a Maasai man found transparent violet-to-blue crystals at Merelani in northern Tanzania and showed them to prospector Manuel d'Souza, who registered mining claims hoping for sapphire. Gemmologists identified the crystals as a variety of zoisite. Tiffany & Co. became the main distributor, named the gem after Tanzania and promoted it with a publicity campaign in 1968, after which it quickly became popular.

In the modern jewellery trade, tanzanite is listed as a December birthstone; the International Gem Society describes it as a modern option that jewellers designated for that month.