This is an article from the GIA library.
Tanzanite is relatively new to the colored stone galaxy. This transparent blue gem first turned up in 1962, scattered on the Earth's surface in northern Tanzania, in eastern Africa. Scientists identified it as a variety of the mineral zoisite. About five years later, a prospector discovered a large deposit of it in the same area, and serious mining began.
Tiffany & Company recognized its potential as an international seller and made a deal to become its main distributor. Tiffany named the gem after the country it came from, and promoted it with a big publicity campaign in 1968. Almost overnight, tanzanite was popular with leading jewelry designers and other gem professionals, as well as with customers who had an eye for beautiful and unusual gems
Tanzanite's public recognition and popularity have grown steadily. But there have been wide fluctuations in the gem's supply and price level, due mostly to Tanzania's volatile political, social, and economic conditions. That country remains the gem's only source.
Tanzanites are heat-treated to produce colors that include light to dark violetish blue and bluish purple, as well as pure blue. Rich, deep hues are valued most, but you'll usually see these only in stones weighing 5 cts. or more. This is mainly because of decisions made during the cutting process. Tanzanite typically shows strong pleochroism, which means it displays different colors from different directions. It usually looks violetish blue from some directions, purplish from others.
Predominately blue tanzanite is generally worth more per carat, but because of the way tanzanite crystals grow, a cutter can usually get a bigger stone by orienting the gem to show the purple color. With small rough, size is normally the main consideration. While the trade considers the pure blue stones to be the "top" grade, some customers actually prefer the lighter and more purplish colors.
Tanzanite is special-care gem for two reasons: sensitivity to thermal shock and the potential for cleavage. Sometimes the temperature change between the hot lights of the display case and the chilly glass countertop in an air-conditioned showroom can be enough to develop cleavages in tanzanite.
Because of its susceptibility to cleavage, tanzanite shouldn't be handled carelessly. Active people should consider jewelry that won't be exposed to as many bumps — pendants and earrings are good choices.
Hardness & Toughness
Sudden changes in temperature may cause cracking in tanzanite. The gem is stable to light, and can be attacked by hydrochloric and hydrofluoric acid.
Tanzanite is routinely heat treated to produce tanzanite color in transparent brownish material. This treatment is undetectable, but is assumed because of its prevalence.
Care and Cleaning
Tanzanite should be cleaned using warm, soapy water. Ultrasonic cleaners and steam cleaners should never be used.
Glass, synthetic corundum, synthetic spinel, and synthetic spinel triplets have all been used to imitate tanzanite.
Text from GIA's Essential Colored Stone Reference Guide © 1999