This is a natural 17.6 carat Tanzanite Crystal
Tanzanite is the Rare blue/violet variety of the mineral zoisite. It was discovered in Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. Tanzanite is used as a gemstone, and naturally-formed tanzanite is extremely rare, still found only in the Mererani Hills.
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.