Umba River (Umba Valley), Korogwe Rural District, Tanga Region, Tanzania

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

Gemstone List

Gem list contains entries from all listed localities in region.

Gemstones from Umba River (Umba Valley), Korogwe Rural District, Tanga Region, Tanzania by Colour

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Localities in this Region

Tanzania
  • Tanga Region
    • Korogwe Rural District
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