A to Z of Gemstones: Tanzanite

TANZANITE

Gemstone family: Zoisite

Crystal System: Orthorhombic

Moh's scale of hardness: 6-7

Lustre: Vitreous

Birthstone: December

About: The origin of the name Tanzanite comes from the country in which it was first discovered, by Tiffany & Co in 1967, and is the only known source of the stone: Tanzania.  Tanzanite belongs to the zoisite species and it comes in striking purplish blue to blue hues.  Normally, tanzanites are very clean stones free from inclusions and they have a characteristic strong pleochroism displaying blue, purple and pale greenish yellow colours.  Most tanzanites’ colour is the result of heat treating zoisite where yellowish and brownish stones become an amazing blue and purplish blue.

Origin: The main and only source of Tanzanite as mentioned is Tanzania in eastern Africa.

Use in jewellery: Tanzanite’s lustre is vitreous; it rates 6-7 on the Mohs scale and has cleavage planes which make it suitable only for jewellery that has a secure setting to protect the stone from knocks.  Tanzanites are usually transparent to translucent and they are cut into faceted gems, cabochons and beads.

Treatments, synthetics and imitations:  Most tanzanites’ colour is the result of heat treating zoisite where yellowish and brownish stones become an amazing blue and purplish blue.