Tanzanite

Tanzanite

A violet-blue zoisite found commercially in only one place on Earth, and remarkable for showing three colours in one crystal.

Overview

Tanzanite is the blue to violet variety of the mineral zoisite, coloured by vanadium. Commercial deposits are confined to a small area of the Merelani Hills in northern Tanzania, making it one of the most geographically restricted of all major gemstones.

The stone is strongly pleochroic: viewed along different crystal directions it can appear blue, violet or bronze-brown. Gentle heating removes the brown component, and the large majority of tanzanite in the trade has been heated in this way.

With a hardness of 6 to 7 and one direction of perfect cleavage, tanzanite calls for careful orientation during cutting. Our work looks at how crystal orientation affects face-up colour and at the features that distinguish heated from unheated material.