Tanzanite is the blue/purple variety of the mineralzoisite (a calcium aluminium hydroxy silicate) discovered in the Mererani Hills of Manyara Region in Northern Tanzania in 1967. 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. It is rare & valuable as a gemstone.
Here are my Tanzanite gemstone digital paintings. Notice the color variation with lighting: