The basically blue variety of zoisite discovered in Tanzania in 1967 is commonly known as tanzanite.



Appearance:
This gem has a characteristic blue color, usually with a violet tinge. In lighter-color specimens, it is almost lavender. This is the principal color, visible from the table facet in cut stones, because another characteristic of tanzanite is strong pleochroism from violet-blue to violet, grayish, or greenish. It has vitreous luster. The stones have few inclusions; where present, these sometime look like thin, parallel tubules. It is normally given a round or oval, mixed cut, but the step cut is also used.



Distinctive features:
The particular color, combine with the type of pleochroism describe and moderate luster, make it fairly easy to distinguish. It can sometimes resemble cordierite, but this has much lower refractive indices and lower density. It is sometimes confused with sapphire, but compare with this, it is visible less luster and much less hard. It has also has different pleochroism and different refractive indices. It is set to have very low resistance to ultrasound. Tanzanite jewellery should, therefore, never be clean with the ultrasonic cleaners commonly used by jewellers and watchmakers. On a number of occasions, stones thus treated have suffered irreparable damage.



Occurrence:
Very limited quantities are found, almost exclusively in Tanzania, where, moreover, the chief deposit is apparently nearly exhausted.



Value:
Given its attractive color, its rarity, and the publicity which greeted its discovery, the value of tanzanite is quite high, little less than of violet-blue sapphires it resembles. But it is rarely seen on the market and is very much a collector’s item. Its modest hardness makes it unsuitable for rings, which are, of course, susceptible to knocks and abrasions.