corundum, sapphire, Umba, Tanzania.12, inclusions.5, gems

The many colours exhibited by corundums from Umba Valley in Tanzania are related to their varying contents of Fe, Ti, Cr and V. Ni and Co could not be detected by the EDS-XRF technique used. The presence of abundant inclusions distinguishes these gemstones, and the following were found: zircon, rutile(?), apatite, hematite, mica, monazite, plagioclase, calcite, pyrrhotine and graphite. Healed fractures, negative crystals, colour zoning, twinning with traces of intersecting lamellae and boehmite(?) laths were noted. Techniques for the identification of inclusions are discussed. The densities (3.98–4.01 g/cm³) and refractive indices (nₑ 1.761–1.768, nₒ 1.769–1.778) were also determined. Spectral features are given.