The Umba River deposit is 70 km northwest of the seaport of Tanga. Corundum occurs in veins of corundum anorthosites within a serpentinite massif in the core of a brachyanticlinal fold. The massif is about 2 km long and 0.5 to 1 km wide.
Corundum crystals have a hexagonal tabular shape without dipyramidal faces (Hanni, 1987). Colour is often zoned, with darker cores in the rubies
, and with darker margins in the sapphires
. Bluish green, yellow-green, brownish yellow, reddish orange (padparascha
type), and violet-red tones are characteristic of the corundums. Some violet (vanadium-bearing) sapphires exhibit the alexandrite effect. The refractive index is 1.761 - 1.778; and the density is 3.96 - 4.01 g/cm³. Crystals contain numerous mineral inclusions.
Ref: E.Ya. Kievlenko, Geology of gems, 2003, p. 59 - 60
Gem list contains entries from all listed localities in region.
Gemstones from Umba River (Umba Valley), Tanga Region, Tanzania by Colour
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Localities in this Region