Tanzanite is a trade name for deep blue to violet zoisite, colored by the presence of trace amounts of the element vanadium. It was first discovered in 1967 in the Merelani Hills of Tanzania and was named and marketed by Tiffany’s. While zoisite is found around the world, and is sometimes even gem-clear, the only location where blue zoisite–tanzanite–is the Merelani Hills of Tanzania. Luckily the deposits are fairly large, so while tanzanite is rare, its plentiful enough to be a significant gemstone from a commercial perspective. Natural tanzanite is almost always a brownish-purple color; its heated to drive out the brownish and yellow tones and turn the stones the rich blue-violet color they are famous for. A very small amount of tanzanite is naturally this color, having been heated underground over the millennia. It’s a trichroic gem, which means that it shows different colors on each of three different optical axes, although heating will change most stones such that they are only dichroic. With its amazing blue color and excellent brilliance (it has a refractive index of 1.685 to 1.707), tanzanite makes a stunning gemstone; however, its relatively soft at 6.5-7 on the Mohs scale, and can easily abrade or chip. A proper mounting and good care can ensure that the gem stays in top condition.
There are no species or varieties of tanzanite.
Where is it found
Tanzanite is only found in the Merelani Hills of Tanzania.
Most tanzanite has been heated to change it from a brownish color to the lovely deep blue to violet-blue the gem is famous for. The change is permanent. Because most tanzanite has been heated, there is little to no premium for unheated tanzanite gemstones.
Use in jewelry
As indicated earlier, the one shortcoming of tanzanite is that its not terribly hard. That means that it should be mounted in a way that can protect the gem from being easily chipped or abraded. Care should be taken when cleaning tanzanite; avoid the use of an ultrasonic jewelry cleaner.
You can look at my current inventory of loose gemstones online here: http://www.earthstreasury.com/product-category/gemstones/tanzanite-gemstones/