corundum, sources.12, Tanzania.12, gems

Tanzania emerged as a great producer of colored gemstones in the second half of this century. Most of the important localities (e.g., Longido, Lake Manyara, Merelani, and Umba) are in the northern part of the country; they produce a great variety of gems, the most impor-tant being tanzanite, corundum (ruby and fancy-color sapphires), garnet (particularly rhodolite and tsavorite), tourmaline, emerald, alexandrite, spinel, and quartz. Diamonds are mined from the Mwadui kimberlite pipe, also in the north. Historically, gemstone occurrences in the south have been rare.. In 1994, however, large and rich gem deposits were found in southern Tanzania in the vicinity of Tunduru, about 80 km north of the border with Mozambique. An estimated 80,000 “illegals” have been working the alluvial deposits in the vicinity of the Muhuwesi and Mtetesi Rivers, mostly by primitive methods. Although the primary sources of the gem minerals have not been found, the fact that some crystals are only slightly rounded indicates short transport distances. This article discusses the history of the discovery and the development of gemstone trade in this area, the geologic setting of the deposits (in the Proterozoic Ubendian-Usagaran System of metamorphic rocks), and the most important gem materials found to date. These include blue and fancy-color sapphires, ruby, many colors of spinel, garnet (rhodolite, hessonite, and color-change), and chrysoberyl (cat’s-eye and alexandrite).PB