corundum, Tanzania.12, gems

Tunduru-Songea gem fields in southern Tanzania.. Dr. Claudio Milisenda of the German Gemmological Association, using research done with Dr. Ulrich Henn and Dr. Hermann Bank, discussed the spectacular new gem occurrences that were discovered in late 1993 and early 1994 in the Tunduru-Songea area of extreme southern Tanzania. These alluvial deposits are located along tributaries of the Ruvuma River, which forms Tanzania’s. southern border with Mozambique. The area produces a large variety of gem materials, in particular blue and fancy-colored sapphire (including “padparadscha” and color-change stones), ruby, many colors of spinel, garnet (such as rhodolite, hessonite, and color-change), chrysoberyl (including cat’s-eye, alexandrite, and cat’s eye alexandrite), various quartz and beryl varieties, tourmaline, zircon, kyanite, scapolite, peridot, and diamond... The first gem rough appeared on the market in 1994, and included sapphire, spinel, garnet, chrysoberyl, and zircon, reportedly from an area east of Songea, where the Muhuwesi and Mtetesi rivers converge. Shortly afterwards, an even richer alluvial deposit was discovered near Tunduru, a remote area in southeast Tanzania. Because thousands of people were involved in mining and trading gemstones at the outset, it is almost impossible now to tell if some rough gems are from the Songea or the Tunduru area. With the exception of the diamondiferous Mwadui pipe within the Tanzanian craton, all of Tanzania’s known gemstone occurrences (including the Tunduru-Songea gem fields) are situated within the Proterozoic Ubendian-Usagaran System, a medium-to predominantly high-grade metamorphic unit that includes gneiss, marble, metapelite, and quartzite, which are intruded by numerous pegmatites. The Ubendian-Usagaran System forms the Tanzanian part of the Mozambique Belt. This gem-bearing complex experienced its main thermo-tectonic evolution during the Pan-African orogeny (approximately 600 million years ago), which affected the entire eastern part of the African continent as well as southern India and Sri Lanka. Dr. Milisenda also reported on the gemological characteristics of corundum, spinel, chrysoberyl, garnet, and tourmaline from the Tunduru-Songea region, which were based on the study of hundreds of carats of rough and cut stones. A visit to the localities yielded information on the geology and recovery techniques. Iron-and titanium-oxide contents of the corundum were compared with similar gem materials from Sri Lanka and India. With the exception of the occurrence of diamond and an unusual mint-green, vanadium-colored chrysoberyl, the gem gravels of the Tunduru-Songea region strongly resemble the gem concentrates of Sri Lanka, and the two areas may be genetically related. However, the large variety of gems of contrasting genetic type suggests that the gems themselves were derived from different geologic sources in both locales.