Connoisseur's Choice: Tanzanite, Gem Variety of Zoisite, Merelani Hills, Simanjiro District, Manyara Region, Tanzania

Tanzanite is one of the few minerals that can be cut into fine gemstones and that also occurs as collectorquality crystal specimens, yet it is restricted to one geological environment in one small geographic region on one continent: Africa. Added to this uniqueness is a continuous global demand for gem tanzanite that has been ongoing for more than fifty years and that has been accompanied by high prices for quality stones since its discovery in 1967. Fortuitously, “African Sky Blue,” is a Johnny Clegg and Savuka song title. Anyone who has had the opportunity to view the African sky on a clear, cloudless day can attest to its blue intensity and vibrancy. It is appropriate then that tanzanite, one of the most attractive blue minerals that to-date is exclusive to Africa, mirrors this blue vista. For its unique color and restricted occurrence, tanzanite from the Merelani Hills in Tanzania is the mineral featured in this Connoisseur’s Column. Tanzanite is not officially recognized as a species by the International Mineralogical Association but is a gem variety of the sorosilicate zoisite, {Ca2}{Al3}(Si2O7)(SiO4)O(OH), a species that occurs in different colors and from many localities worldwide; however, only tanzanite is dark blue-violet, the cause being the presence of traces of vanadium V3+, as well as chromium and strontium. The most sought after tanzanites are the ones with the greatest blue saturation. Zoisite is the orthorhombic polymorph of clinozoisite (Armbruster et al. 2006; Mills et al. 2009), which occurs as prismatic crystals commonly less than 10 cm in length, but larger crystals are known, some of which are shown here. It has perfect {010} cleavage, 6–7 hardness, and is typically translucent to transparent (Anthony et al. 1995). Wilson et al. (2009) describe a wide variety of colors for zoisite other than the blue-violet of tanzanite. These include shades of brown, yellow, yellow-green, green, green-brown, turquoise-blue, bluish-purple, purple, pink, red, red-brown, gray, and colorless; color zoning in individual specimens is common. The moniker tanzanite has been generally and incorrectly used for various colored zoisite crystals from Tanzania other than blue-purple-violet, and this should be avoided (Gübelin 1992). Mindat.org lists more than one thousand global localities with zoisite, and images on the site display the variety of colors listed above. Tanzanite is trichroic, displaying blue, red, and yellowbrown-green when viewed down different crystallographic axes (“phenomenal trichroism,” Crowningshield 1967, p. 202). Heat treatment between 400°–650°C to remove the undesired green-brown color renders the tanzanite dichroic; a red tint seen looking down the c-axis is a good indication that the crystal coloration is natural. Many original “tanzanite” crystals are initially yellow-green-brown zoisite, although natural, unheated blue tanzanite is found, but it is relatively rare. Therefore, a significant percentage of tanzanite crystals and gems on the market have been heat-treated (Hurlbut 1969; Sundberg and Valkonen 1988; Vitali-Herbert Gem Variety of Zoisite Merelani Hills, Simanjiro District Manyara Region, Tanzania