tanzanite (tănzăn´īt), 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°C, 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.