Tanzanite is the blue/purple variety of the mineral zoisite (a calcium aluminium hydroxy silicate) which was discovered in the Mererani Hills of Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. It is used as a gemstone. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet and burgundy depending on crystal orientation. Tanzanite can also appear differently when viewed under alternate lighting conditions.

The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination. Tanzanite in its rough state is usually a reddish brown color. It requires artificial heat treatment to 600 °C in a gemological oven to bring out the blue violet of the stone. Tanzanite is a rare gem. It is found mostly in the foothills of Mount Kilimanjaro. The mineral is named after Tanzania, the country in which it was discovered.

Source: en.wikipedia.org/wiki/Tanzanite

Cause of Color :


Chemical Composition :

Silicate of calcium and aluminium – Ca2(Al,OH)Al2(SiO4)3

Classification / Type :

Belongs to the epidote group of minerals.

Crystal System / Forms :

Orthorhombic System

Cuts & Uses :

Facetted cuts, cabochon, carving, etc.

Dispersion : 0.021
Hardness : 6
Lustre : Vitreous
Magnification :

Parallel fibrous canals, crystals, fingerprints.

Optic Character :

Anisotropic, D.R.; Biaxial positive.

Pleochroism : Deep blue / purple / pink; or, reddish purple / blue / yellowish brown
Refractive Index / Birefringence :

1.691 – 1.700 / 0.009; Range: 1.688 – 1.707

Simulants (with separation tests) :
  • Synthetic Forsterite: Strong pleochroism (similar to tanzanite), R.I. 1.635 – 1.670, strong doubling, dotted gas bubbles.
  • Synthetic Blue 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)

Sources :


Specific Tests :

Strong pleochroism

Spectrum :

Absorption centered at 595nm

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