gems, zoisite, tanzanite, Merelani, Tanzania, East Africa, Mozambique belt, geology, age dating

Seven crystals of tanzanite from the Ally and D’Souza Mines, Tanzania, gave an average fission track age of 585 ± 28 million years. Tanzanite, a gem variety of zoisite, has annealing characteristics very similar to epidote; both require a temperature above 725 °C for complete track annealing within an hour. Sub-gem-quality tanzanite from a new occurrence 70 km to the south, in or near the Uluguru Mountains (Tanzania), has a younger age of 355 million years.