tanzanite

{{Infobox mineral |category = Sorosilicate: zoisite variety |boxwidth = |boxbgcolor = #cba0fa |image = Zoïsite (Tanzanite).jpg |imagesize = 260px |caption = |strunz = 09.BG.10 |formula = |color = Royal blue, indigo, violet/purple |habit = Prismatic crystals with striations; massive to columnar |system = Orthorhombic |symmetry = Pnma (no. 62) |twinning = penetration twins |cleavage = Perfect {010}, imperfect {100} |fracture = Uneven to conchoidal |mohs = 6.5 |luster = Vitreous, pearly on cleavage surfaces |refractive = 1.69–1.70 |opticalprop = biaxial positive |birefringence = 0.006–0.018 |pleochroism = Present, dichroism or trichroism, depending on heat treatment |streak = White or colorless |gravity = 3.10–3.38 |density = |fusibility = |diagnostic = |solubility = |diaphaneity = |other = }} Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate). The color is caused by small amounts of vanadium. Tanzanite belongs to the epidote mineral group. Tanzanite is only found in Simanjiro District of Manyara Region in Tanzania, in a very small mining area approximately long and wide near the Mererani Hills.

Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation. Tanzanite can also appear differently when viewed under different lighting conditions. The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination. In its rough state tanzanite is coloured a reddish brown to clear, and it requires heat treatment to remove the brownish "veil" and bring out the blue violet of the stone.