gems, garnet, grossular, yellow grossular, Tanzania

Tanzanian yellow grossular garnets. Although best known for its hessonite and tsavorite varieties, grossular garnet occurs in a wide range of colors. Recently, private collector Don Clary submitted to GIA’s Research Department three stones from Tanzania for investigation. The largest of the three stones, a 3.17-ct barion cut (figure 8), was studied in detail to understand the origin of its unusual color. EDXRF spectroscopy detected the presence of titanium, manganese, and iron. UV-visible absorption spectroscopy revealed increasing absorption toward the ultraviolet, with three sharp lines at approximately 407, 418, and 428 nm, and a doublet at about 370 nm. These sharp bands are typical of Mn²⁺ in the distorted cubic site in garnets. It was concluded that the yellow color of these Tanzanian grossularite garnets is primarily due to manganese (Mn²⁺), with no significant contribution from iron.