<Tr> <Th> Birefringence </Th> <Td> 0.006--0.018 </Td> </Tr> <Tr> <Th> Pleochroism </Th> <Td> Present, dichroism or trichroism depending on heat treatment . </Td> </Tr> <P> Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate) belonging to the epidote group . The gemstone was discovered by Manuel d'Souza in the Mirerani Hills of Manyara Region in Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro . Tanzanite is only found in Tanzania, in a very small mining area (approximately 7 km long and 2 km wide) near the Mirerani Hills . </P> <P> Tanzanite is noted for its remarkably strong trichroism, appearing alternately 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 . In its rough state tanzanite is usually colored a reddish brown, and much of it requires heat treatment to remove the brownish "veil" and bring out the blue violet of the stone . </P>

Tanzania is the only country in the world with a unique gemstone called