Garnet, corundum and other gem minerals from Umba, Tanzania
Both rhodolite and almandine garnets occur in Umba. The rhodolites have no special properties by which they can be distinguished from other garnets in the pyrope-almandine series, except their rose-red colour. Hence a distinctive name is unnecessary. Rutile appears to be the most frequent mineral inclusion in rhodolite, whereas in almandine both apatite and rutile are common. Corundums of different colours occur. Green corundums owe their colour to iron, while blue corundums have a high Ti and a low Cr content. Common inclusions are rutile, pyrrhotite and apatite, whereas graphite especially is found in deep violet corundums. The inclusions in both the garnets and the corundums are not characteristic of this locality. The properties of emerald green and orange-brown tourmalines are given, as well as those of brownish orthopyroxenes, a light green clinopyroxene, yellow scapolite, reddish brown zircon and turquoise.
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Zwaan, P.C. (1974). Garnet, corundum and other gem minerals from Umba, Tanzania. Scripta Geologica, 20, 1–41.