Color: Sapphire blue, amethyst, violet
Hardness: 6.5 - 7 Mohs
Specific Gravity: 3.35
Refractive Index: 1.691 - 1.700
The name Tanzanite was introduced by the New York jewelers Tiffany & Co after an East African state called Tanzania. The name is accepted by the gems trade, but scientist refer to it as Blue Zoisite. We have to thank Tiffany & Co for disliking the name 'Blue Zoisite' which sounds pretty much like 'Blue Suicide'.
Where do we get Tanzanite gems
Obviously in Tanzania we can see the mines operating to get tanzanite gems. Nowhere else. However, the government of Tanzania banned the export of rough stones weighing more than one gram. Most likely you will see the fine, polished tanzanite gems when you visit your local gems store.
Most (if not all) tanzanites used in jewelry industry are heat treated. The temperature is usually between 730-390°C for 30 minutes, and the stones should not have any cracks or bubbles, as they could shatter or the cracks/bubble could increase in size during furnace heating. (Ref: Wikipedia)
Recently, coated tanzanites were discovered and tested by the AGTA and AGL laboratories. A thin layer containing cobalt had been applied to improve the color. It was noted that "coatings in particular are not considered permanent", and in the United States are required to be disclosed at the point of sale.