Tanzanite

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

Tanzanite 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
Crystal System / Forms: 
Orthorhombic System
Hardness: 
6
Specific Gravity: 
3.15 - 3.38
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
Pleochroism: 
Deep blue / purple / pink; or, reddish purple / blue / yellowish brown
Dispersion: 
0.021
Magnification: 
Parallel fibrous canals, crystals, fingerprints.
U.V. Fluorescence: 
Inert.
Spectrum: 
Absorption centered at 595nm
Cause of Color: 
Vanadium
Treatment (Enhancement): 
  • Colored impregnation: Blue color is introduced into fractures to improve color.
  • Heat treatment: Commonly brownish blue stones are heated to produce the deep blue of tanzanite. The intensity of the starting color decides the percentage of the color change.
Specific Tests: 
Strong pleochroism
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)
Geological Occurrence: 
In metamorphic rocks subjected to regional metamorphism.
Sources: 
Tanzania
Cuts & Uses: 
Facetted cuts, cabochon, carving, etc.