I have found this definition of tanzanite to be succinct, yet all-encompassing.
tănzănˈīt, 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 degrees Celsius, 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.
From Questia.com Online Library.