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. Tanzanites are extremely rare to find because it only comes from one mine and has high demand. Tanzanites were discovered in 1967 and were given the name by Tiffany & Co. It is found in the Usumburu Mountains that border the Umba Valley of northern Tanzania.