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What is Tanzanite?

Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate), caused by small amounts of vanadium.[3] Tanzanite belongs to the epidote mineral group. Tanzanite is only found in Tanzania, in a very small mining area (approximately 7 km (4.3 mi) long and 2 km (1.2 mi) wide)[4] near the Mererani Hills.[5]

Tanzanite
Zoïsite (Tanzanite).jpg
General
Category Sorosilicate: zoisite variety
Formula
(repeating unit)
(Ca2Al3(SiO4)(Si2O7)O(OH)) + (Cr,Sr)
Strunz classification 09.BG.10
Crystal system Orthorhombic[1]
Space group Pnma (no. 62)
Identification
Color Royal blue, indigo, violet/purple
Crystal habit Prismatic crystals with striations; massive to columnar[1][2]
Twinning penetration twins
Cleavage Perfect {010}, imperfect {100}[1]
Fracture Uneven to conchoidal[1]
Mohs scale hardness 6.5
Luster Vitreous, pearly on cleavage surfaces
Streak White or colorless
Specific gravity 3.10–3.38
Optical properties biaxial positive
Refractive index 1.69–1.70
Birefringence 0.006–0.018
Pleochroism Present, dichroism or trichroism, depending on heat treatment

Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation.[6] 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 colored a reddish brown to clear, and it requires heat treatment to remove the brownish “veil” and bring out the blue violet of the stone.[7]

The gemstone was given the name ‘tanzanite’ by Tiffany & Co. after Tanzania, the country in which it was discovered. The scientific name of “blue-violet zoisite” was not thought to be sufficiently consumer friendly by Tiffany’s marketing department, who introduced it to the market in 1968. In 2002, the American Gem Trade Association chose tanzanite as a December birthstone, the first change to their birthstone list since 1912.[8]

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