gems, tanzanite, zoisite, chromophores, spectra

Zoisite and especially its blue-to-violet variety tanzanite are popular gemstones. While the mineral is found in several places around the world, gem quality material mostly comes from a region southeast of Mount Kilimanjaro in Tanzania. Naturally blue to violet material is rather rare but a simple heat treatment can convert much of the off-colored, yellowish or grayish rough, which represents the majority of the crystals unearthed, to violet or blue colors of various tones and saturation. The source of violetish blue color are small amounts of vanadium substituting for aluminum but little is known that zoisite can also be found in other attractive colors, such as yellow, green, pink or even colorless. In this article we investigate zoisites from Merelani, Tanzania and correlate their colors with the presence and concentration of trace elements, as well as the changes in color after heat treatment. Vanadium in blue, chromium in green, manganese in pink to red and an iron titanium intervalence charge transfer complex in yellow zoisites have been found to be the most important chromophores but also rare earth elements such as neodymium seem to play a role in coloration when present in larger amounts, e.g. in orange stones.