Ruby and sapphire deposits of the anorthosite type are known in the Umba River Valley and in the region of Longido in northern Tanzania, near the Kenyan border. They are bodies of metamorphosed ultramafic rocks, which are part of the older Precambrian Usagaran group of granite gneisses, crystalline schists, quartzites, and dolomites within the Mozambique foldbelt.
Ref: E.Ya. Kievlenko, Geology of gems, 2003, p. 59
Gem list contains entries from all listed localities in region.