corundum, ruby, Tanzania.12, gems, Winza

The authors present the results of a characterization study of gem rubies and sapphires from a relatively new locality near the village of Winza in central Tanzania. Since high-quality rubies were accidentally discovered in this remote region in the Fall of 2007, artisanal miners have recovered stones at the surface or in vertical shafts from an area covering several square kilometers. This report begins with a summary of the region’s geology. The rubies occur in association with amphibolites as rhombohedra or as tapered prismatic euhedral and subhedral crystals up to several centimeters long in a garnet-pargasite host rock. Weathering of the host rock near the surface distributed loose corundum crystals in the soil. Most are red to purplish red (occasionally violet or blue). In many instances, the corundum displays internal color zones with banded or irregular areas that are blue, pink, or both. Solid inclusions identified include apatite, chlorite, garnet, pargasite, pyrite, spinel, and talc. Whitish particle clouds and both primary and secondary fluid inclusions are also common, as is crystallographic growth zoning. Especially distinctive in Winza rubies are flat or needle-like inclusions, the latter exhibiting unusual curved or spiral patterns in some cases.. Minor and trace-element data obtained by LA-ICP-MS analysis showed small amounts of Ni (up to ~25 ppm), an unusual trace element in natural corundum. Multiple analyses along the traverses of color-zoned crystals correlated trace-element contents with coloration. Heating experiments on Winza rubies using traditional techniques were not very successful, producing orange coloration and diminishing transparency. Heat treatment appears to be detectable by infrared spectroscopy.