The ruby-bearing amphibole-zoisite-plagioclase veins are in amphibolites of the Usagaran group near the town of Mdarara, about 100 km northwest of Mount Kilimanjaro. The ruby-like corundum
in a bright green matrix of chromian zoisite and pargasite is mined for decorative, collection, carving material and rarely for cutting cabochons.
The flattened hexagonal crystals of pink and violet-red corundum
, up to 30 cm in diameter, contain rare translucent areas, which increase the decorative value of the stone. Pargasite, spinel and zoisite were identified as solid inclusions in corundum (Themelis, 1989). The indices of refraction are 1.764, 1.772 and the density is 3.99 g/cm³.
Ref: E.Ya. Kievlenko, Geology of gems, 2003, p. 60