Tanzanite is a trade name that was first used by Tiffany and Company for gem-quality specimens of a mineral named zoisite with a blue color. Tiffany could have sold the material under the mineralogical name of “blue zoisite,” but they thought the name “tanzanite” would stimulate customer interest and be easier to market.
The name “tanzanite” was given because the world’s only known tanzanite deposit of commercial importance is in northern Tanzania. The name reflects the gem’s limited geographic origin. The mines are all located in an area of about eight square miles in the Merelani Hills, near the base of Mount Kilimanjaro and the city of Arusha.
The blue color of tanzanite is caused by small amounts of vanadium within the zoisite mineral structure. When vanadium-bearing zoisite is heated to a temperature of 600 degrees Celsius for about 30 minutes, the oxidation state of the vanadium is changed and that change causes or improves the blue color.
Today, nearly all of the gems being sold as “tanzanite” have a blue color that has been produced or enhanced by heating. A small amount of tanzanite in the marketplace has a blue color that was produced naturally through the heat of metamorphism without any treatment by people. This naturally blue, untreated tanzanite, is held in very high regard by some gemstone and jewelry buyers who seek it out when making a purchase.
Tanzanite has a hardness of about 6.5 on the Mohs Hardness Scale. This hardness is low enough that the gem is vulnerable to being scratched during normal wear if it is used in a ring. This problem can be reduced if the setting is designed to protect the stone from impact and abrasion, or if the ring is not worn during activities when the chance of impact or abrasion is high.
Tanzanite is one of the best-selling colored stones in today’s gem and jewelry markets. That is a surprise considering that it was only discovered in the 1960’s, while all of the other best-selling colored stones have been known for centuries. It has a unique blue color that grows in popularity as more people learn about it.
At the same time, tanzanite is a rare gem. All of the known deposits are confined to a few square miles of land in northern Tanzania. It is the only gemstone with a large and growing popularity that has such a limited known supply. Some people believe that the currently known tanzanite resource could be depleted in just a few decades.