It is said the geology in which tanzanite was born was little short of a miracle, and the Tanzanite Foundation estimates the chance of the same geological phenomenon occurring elsewhere in the world is one in a million. This means the tanzanite mine is the only one in the world and probably always will be, making the gem all the more precious. The gemstone was scientifically called blue zoisite at the time of its discovery, until Tiffany & Co. decided to commercialize it and make it famous by naming it tanzanite. The company decided the name was more ‘sellable’ than blue zoisite.
The tanzanite mine is divided into blocks lettered A through D, with Blocks B and D reserved for the local miners. In 2005, Block C was bought from a company named Tanzanite One, which discovered the biggest tanzanite to date—a magnificent rough weighing 16,839 carats. Tanzanite One also came up with an easy-to-use grading system for tanzanites in the market, which can be found online.
Most of the blue gemstones found in the early days of the mine were said to have been coloured by a wildfire in the area, which heated them naturally if they were close to the surface. The mineral zoisite was discovered in 1805—long before the blue variety was encountered—and can be found in different colours in countries such as Australia and Namibia, but at the time of this writing, the blue variety called tanzanite can still only be found in Tanzania near Arusha.
As mentioned, most tanzanite gems are heat treated as soon as they are out of the mine. The treatment is considered normal and does not have any effect on the gem’s value (though if a natural unheated blue tanzanite is found, that drives the price higher on the market, as with sapphire and rubies).
Tanzanites are not known to be subjected to other treatments—another reason the gem is appreciated in the market. Though there are a few exceptions, they are rare and often disclosed with the sale.