Zoisite gemstone Information

by Manisha G



This mineral has long been known in two opaque ornamental forms. Thulite, which is pink in colour and used ocassionally in stone-ware, boxes etc., and a bright green granular variety found as matrix rock containing large crystals of ruby in the Matabutu mountains of Tanzania.

In 1967, a completely new variety was discovered, also in Tanzania, which provides very fine transparent blue and violet-blue gems of outstanding beauty and importance, and often of quite considerable size. Since the finest of these stones provide gems which rival find sapphire in appearance and colour. There is little doubt that inspite of lower hardness, it is gem of some importance and is therefore included in detail. As it was originally found in Tanzania, it was named as “Tanzanite”.

Variety: Tanzanite: blue, Thulite: pink, Anyolite: green with translucent rubies.

Chemical composition: Calcium Aluminium silicate Ca2(Al OH) Al2 (SiO4)3.

Colour: Fine blue to violet blue (possibly impproved by heating). Some brown crystals have been found which are turned blue by heat treatment.

Physical Properties: 

Cleavage: Perfect, parallel to micropinacoid.

Hardness: 6

S.G.: 3.35 (blue), 3.28 (green), 3.10 (pink).

R.I.: 1.691 – 1.700, D.R. 0.009, Positive sign.

Dichroism: Very strong, sapphire – blue, blue and sage green.

Other important points:

Spectrum: Broad bands at about 5950A and fainter at 4550. Weak lines have been seen in the red. Colour may be due in part to vanadium.

Detection: Could be confused with fine sapphire or an unusually fine blue spinel. The refraction is sufficiently dissimilar to either of these stones to make confusion unlikely once tested. Again the refractive index is completely different.

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