violet-blue gemstone, 1968, named by Henry B. Platt, vice president of Tiffany & Co., because the stone was discovered in the African nation of Tanzania.

Tanzanite is a variety of zoisite. It has naturally occurring shades of blue, green, yellow, pink, brown, and khaki, but virtually all gem-quality crystals are heat treated to produce the highly valued shades of blue and blue-violet. On Mohs' scale of hardness, tanzanite is 6.5 to 7. So far, the single source of the world's tanzanite is i [..]