Chemical composition -- Hydrated calcium aluminum silicate.
Color -- Blue, violet, yellow-brown, deep green (chrome).
Almost all tanzanite is heat-treated after cutting to enhance
the blue-violet coloration and eliminate yellow-brown components. The color change is
permanent. Scarce chrome tanzanite (chrome zoisite) obtains its vivid green color from
chromium content.
Optics -- R.I. 1.69-1.70.
Durability -- Hardness 6-7. Brittle, with one direction of perfect cleavage. Although heavily promoted by the jewelry industry for widespread use, tanzanite is really too fragile for use in rings that are worn daily, where it can quickly become scratched and chipped. Careful use in dinner rings with protective settings is acceptable. It is best suited for items less subject to rough wear, such as earrings, pendants, and pins.
Tanzanite should never be exposed to ultrasound, as it has been known to fracture or even shatter in ultrasonic cleaners.
Crystal structure -- Orthorhombic .
Specific Gravity -- 3.35.
Sources -- Tanzania is the only known source. Chrome tanzanite, an intense green variety discovered only a few years ago, is very rare and found only in fairly small crystals. Large, fine tanzanites can be quite costly. In 1996-97, mining was rather intense, and large quantities of tanzanite hit the market, dropping prices sharply. Early in 1998, however, severe rains flooded the primary mines and killed more than 100 miners. Prices rebounded as the supply diminished and rose significantly for a couple of years before moderating as mining resumed and supplies were restored.