Tanzanite: Gemstones Information
Probably the most important in the gem context of the epidote group minerals is zoisite Ca2Al3(SiO4)(Si2O7)O(OH) as this includes the magnificent transparent blue material known universally by the varietal name tanzanite. Tanzanite is especially notable for its pleochroism which in one specimen I (MO’D) have examined shows the now familiar deep blue in one direction but also ruby red and emerald green in the other two directions.The pleochroic green is replaced by rich gold in many specimens. As well as tanzanite whose occurrence is till apparently limited to Tanzania, a transparent green zoisite has been found in the northern areas of Pakistan.Tanzanite is very brittle and should be handled with care; it should never be placed in ultrasonic cleaners. The hardness is 6–7, SG is 3.35, the RI 1.692, 1.693 and 1.700 for the alpha, beta and gamma rays respectively,biaxial positive with DR about 0.009. Tanzanite occurs in calcareous rocks, including metamorphosed dolomite and calcareous shales subjected to regional metamorphism. Occasional reports have claimed other localities for tanzanite but so far finds have been confined to the Lalatema area of Tanzania. A recent study by Valerio Zancanelli, Tanzanite, 2004; ISBN 8890150904, includes some useful and interesting information.