Tanzanian Ruby: Gemstone Information
Corundum from Tanzania occurs in a wide range of colours including orange, purple, mauve and yellow as well as red, green and blue. In general, the colours can be said to be pastel rather than well defined but are particularly attractive for this reason. Ruby inclusions in a bright green chromiferous rock make good carving material and have been given the vernacular name anyolite. Of greater importance are the varieties of sapphire from the Umba river valley in north-eastern Tanzania, near the Kenya border where sapphires are mined from several pits in and around a greyish green serpentine pipe. The sapphires and rubies of Umba occur in various colours, often of a pastel shade which is very attractive. Purple and mauve and orange sapphires, in particular, are beautiful.
In the Fall 1991 issue of Gems & Gemology, Hänni and Schmetzer report the occurrence of ruby in parcels of red spinels said to be from the Morogoro area of Tanzanian Ruby. The colour and inclusions were similar to some found in Myanmar ruby; inhomogenieties in growth zoning parallel to distinct faces and irregular swirls; sets of twin lamellae parallel to one, two or three rhombohedral faces; clouds of rutile crystals and coarse rutile; octahedral inclusions with slightly rounded solid material (spinel) or negative crystals looking similar. Zircon, apatite and pyrope have also been identified. The potential for asterism has been found in some specimens.
The mines are situated in the western Uluguru Mountains. An excellent account of ruby mining in Tanzania appeared in extraLapis 15, Rubin, Saphir, Korund, 1998. See also zwaan garnet, corundum and other gem minerals from Umba in Scripta Geologica.