Is a variety of Zoisite, a Calcium Aluminum Silicate Hydroxide. The formula is
Ca2Al3(SiO4)3(OH) and hardness 6.5-7.
Found in Tanzania in 1967 ( hence the name ) is one of the major gemstone who will keep and increase it’s value in the future.
Most of the tanzanite in the market are heat treated: the starting natural color is brownish,greenish,yellowish or in between; some of them are found already blue-violet, and for those there is no need of heat treatment.The heating is performed at around 500 C° and is permanent. Pleochroism is very strong: when seen from different angles, the color change from blue to violet and sometimes when can observe three different colors at the same time,depending on how the table were oriented on the C axis when the stone was cut. The only know source is Merelani, in the Arusha region,not far from the Kilimanjaro, Tanzania.