tanzanite [key], beautiful gemstone discovered in 1967 in the Umba Valley near the Usambara Mts. in Tanzania, a precious variety of the mineral zoisite, a calcium aluminum silicate. Zoisite is a common rock-forming mineral and is usually white to gray in color. Tanzanite occurs as orthorhombic crystals, which may be colorless, yellow-green, brown, or blue to violet when found; when these crystals are heated to 300–400℃, many of them turn sapphire blue, which is the preferred color for gemstones. The blue color is attributed to the presence of small amounts of vanadium.
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