Zoisite

Zoisite occurs in a number of varieties, the most sought-after being tanzanite, a variety colored sapphire-blue by the presence of vanadium. Tanzanite crystals have distinct pleochriosm, showing either purple, blue or slate-grey depending on the angle they are viewed from. There may also be a slight color change in incandescent light (such as that from a light blub), when stones may appear more violet. A massive green variety of zoisite, containing rubies and occasionally dark horn-blende inclusions, may be polished, carved, or tumbled to make ornaments or an attractive decorative stone. Thulite, a massive, pinkish red variety colored by manganese, is also polished or carved to make small ornaments. Tanzanite has been confused with sapphire, and thulite with rhodonite. Some heating of zoisite varieties may enhance their colour.

Tanzanite was first found in Tanzania (hence the name). Yellow and green zoisite occurs in Tanzania and Kenya. Thulite is found in Norway, Austria, western Australia, Italy and North Carolina (USA).

Discovered by Baron von Zois in the Sau-Alp mountains of Austria, zoisite was first called saualpite.

Zoisite Pictures

Zoisite beads with other gemstone beadsZoisite with ruby

Zoisite Properties

Chemical Composition: 
Silicate of calcium and aluminium - Ca2(Al,OH)Al2(SiO4)3
Classification / Type: 
Belongs to the epidote group of minerals.
Colors / Varieties: 
  • Blue - Tanzanite
  • Pink - Thulite
  • Green, yellow, brown, violet
  • Chatoyant varieties
Crystal System / Forms: 
Orthorhombic System / Prismatic or tabular crystals, often striated, Also massive.
Hardness: 
6
Cleavage / Fracture: 
Perfect 1 directional cleavage / Conchoidal fracture.
Optic Character: 
Anisotropic, D.R.; Biaxial positive.
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.691 - 1.700 / 0.009; Range: 1.688 - 1.707
Dispersion: 
0.021
Magnification: 
Parallel fibrous canals, crystals, fingerprints.
U.V. Fluorescence: 
Inert.
Cause of Color: 
Green - Chromium
Treatment (Enhancement): 
Colorless impregnation: With oil or resin to improve clarity.
Simulants (with separation tests): 
Synthetic Sapphire (optic figure, R.I., S.G., inclusions), Synthetic Spinel (optic character), Synthetic Quartz (optic figure, R.I., S.G., pleochroism), Iolite (R.I., S.G., pleochroism), Glass (optic character), Tourmaline (optic figure, R.I., birefringence), Rough Smoky Quartz (S.G., pleochroism)
Geological Occurrence: 
In metamorphic rocks subjected to regional metamorphism.
Sources: 
Tanzania, U.S.A. (North Carolina), Australia.