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

Hänni investigates the unusually wide range of corundum colors from the Umba Valley of Tanzania and relates them to variations in trace-element composition. EDS-XRF analyses show that the colors are principally controlled by Fe, Ti, Cr and V, while Ni and Co were not detected. The stones contain abundant inclusions, including zircon, rutile(?), apatite, hematite, mica, monazite, plagioclase, calcite, pyrrhotite and graphite, together with healed fractures, negative crystals, color zoning, twinning with intersecting lamellae, and possible boehmite laths. Hänni discusses methods for identifying these inclusions and reports densities of 3.98–4.01 g/cm³ and refractive indices of nₑ 1.761–1.768 and nₒ 1.769–1.778, together with spectral characteristics. The extraordinary colors—including bluish green, violet, pink, yellowish green, brownish yellow and orange—are contrasted with more conventional ruby and sapphire colors.