A - Z Gemstone Menu

Tanzanite Gemstone Information

Tanzanite was discovered in the Merelani Hills (Mererani) of Tanzania, East Africa in the 1960's. It is a variety of Zoisite that can occur naturally in blue to violet colours, but the majority of gems on the market are the result of heat treating brownish coloured zoisite. Intense colours rarely occur naturally, most highly saturated tanzanite seen in books and on-line images has been heat treated to achieve those outstanding intense colours.

Tanzanite is available in light violet, to blue, to a deep violetish-blue colours. The gemstone has strong trichroic pleochroism, so that blue stones appear violet when turned and flashes of red-violet or yellow-green may also appear through the facets.

Zoisite occurs rarely in yellow, orange, pink, green and brown colours, marketed as yellow Tanzanite, orange Tanzanite etc. They are sold with an emphasis on their rarity to attract the collectors market, where whole or fragmented crystals of Tanzanite in fancy and blue/violet colours are popular. Fine specimens can be found and normally appear on the market at typically 10 carats to 30 carats and over.

Source location for Tanzanite:

There is a limited supply of Tanzanite and the only known commercial deposit is currently in the Merelani Hills (Merelani) of the Manyara region of Northern Tanzania. Close to the Arusha National Park and around 50 km from the Mount Kilimajaro National Park. The mining area is a long strip of land, around 2Km wide and 50Km long.

Zoisite is found in other regions of the world, but blue colours of significant quality or quantity rarely appear.

Physical Properties

Stone type:
Tanzanite is a variety of zoisite.
Crystal System:
Orthorhombic.
Chemical Composition:
Calcium aluminium silicate, Ca2Al3(SiO4)3(OH)
Colour:
Light violet, blue to a deep rich purple. There are also rare fancy coloured tanzanites (zoisite) in yellow, green, pink and other blue shades.
Lustre:
Vitreous.
Pleochroism:
Trichroic strong: Purple, blue and greenish to brownish yellow.
Dispersion:
Low.
Hardness:
6½ on the Mohs' scale of hardness.
Toughness:
Poor.
Cleavage:
Perfect.
Density in gm/cc:
3.15 to 3.38
Double refraction:
Yes.
Refractive index:
1.69 to 1.70

Common Treatments

Natural blue stones may be heat treated to improve the colour, undesirable yellow and brown tints are removed during this process.

Brown stones are heat treated to produce blue to violet stones, often with intense colours


Top of Page | Gem-Guide Home