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.

The heat treatment of tanzanite is very mild when compared to what is often done for gems such as rubies and sapphires. Those gems can be heated to temperatures between 1000 and 1800 degrees Celsius and held at those temperatures for days or weeks.

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.