18K white gold. TCW of Tanzanites: 6.00 ct. Round cut Tanzanite set in prong setting. TCW of diamonds: 1.00 carats, SI clarity and G color, RBC. All stones set in prong setting. Tanzanite is a trade name that was first used by Tiffany and Company for gem-quality specimens of the mineral zoisite with a blue color. The name tanzanite was given because the world's only known tanzanite deposit of commercial importance is in northern Tanzania. The name reflects the gems 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.