corundum, sapphire, Umba Valley, Tanzania.12, East Africa, gems

Rutland describes corundum from newly worked deposits on the upper Umba River in Tanganyika and amethyst reportedly originating in the same country, examining specimens supplied by the gem dealer Charles Mathews following a visit in 1962. The corundum mines lie in bush country approximately fifty miles inland from Tanga, where the mineral occurs in vermiculite and can readily be separated by hand. Rough material shows little evidence of water wear and consists mainly of broken fragments, although tabular hexagonal crystals also occur. Many specimens contain internal fractures, but some display excellent transparency. Rutland suggests that the extensive area of the workings could support substantial production, without providing reserve estimates or production figures. Colours include red, blue, purple, and various intermediate shades, with frequent zoning from the outer prism faces towards the crystal centre. Examples combine blue margins with yellow interiors or pale brown margins with dark red centres. Some richly purple stones become markedly redder under tungsten illumination. Particularly strong dichroism, often visible without an instrument as the rough is rotated, produces pronounced changes such as pale pink to dark magenta or yellow to brown, creating challenges for cutters seeking the most favourable colour orientation. Microscopic observations include conspicuous twin planes, twinning visible between crossed polarizers, long irregular-edged tubes, crystal aggregates resembling those found in Burmese ruby, scattered rounded crystals, and short parallel tubes. Surface features include large and small trigons on the pinacoid and elongated ridges tentatively compared with the internal tubular structures. Measured refractive indices of approximately 1.76–1.77, estimated birefringence of 0.008, and hydrostatic specific gravities slightly below 4.00 agree with ordinary corundum properties. The amethyst was purchased in Dar es Salaam, but its precise locality could not be established; its Tanganyikan origin rests on information given to Mathews. Broken pieces and a doubly terminated crystal show richly coloured interiors enclosed by translucent milky quartz. The better material has a strong, pleasing colour with a distinct red component, carried by the ordinary ray, while the extraordinary ray appears paler and cooler. Some specimens contain abundant fine fibres identified as rutile, arranged in radiating sprays and bundles, occasionally with bent or geniculated forms. These inclusions are inconspicuous to the unaided eye except when reflected light produces a sheen. Rutland suggests that thin slices might yield attractive ornamental “hair stones.” The illustrated account records distinctive colour, growth, and inclusion features while distinguishing the documented Umba corundum locality from the uncertain provenance of the amethyst.