corundum, ruby, sapphire, Tanzania.12, East Africa, inclusions, gems

Bank surveys the principal gemstones then known from Tanzania and relates them to their geological occurrences and mineralogical properties. Following an outline of Tanzanian geology, he treats diamond, garnet, corundum, zoisite, prase opal, diopside, and chromium-bearing tourmaline. Garnets include pyrope–almandine “rhodolite,” hessonite, yellow-green and colorless grossular, and rare emerald-green Cr-bearing grossular from the Mirarani Hills. Corundum is described from the Longido zoisite-amphibolite, where large ruby porphyroblasts show pronounced zoning, parting, rutile needles, and other inclusions; additional Tanzanian deposits yield ruby, sapphire, padparadscha, and fancy-colored corundum. Particular attention is given to transparent blue zoisite from the Mirarani Hills, discovered in 1967, including its pleochroism, optical properties, trace elements, heat-induced color modification, color change, and occasional chatoyancy; its occurrence is described in pegmatitic-hydrothermal veins or fracture fillings with quartz, feldspar, grossular, and diopside. The paper also records nickel-colored green prase opal from Hanety Hill, yellowish-green gem diopside, and emerald-green Cr-tourmaline from the Lelatema Hills, providing refractive indices, densities, lattice constants, trace-element data, and other mineralogical properties for many of the materials.