Sapphire-like corundum, including rare gem quality varieties, is found in nepheline-bearing syenites-pegmatites (miaskites).
Corundum-bearing bodies are apophyses of pegmatite veins, in which the mineral composition, arfvedsonite, aegirine, and lamprophyllite diminishes with distance from the main body, while corundum, hercynite, biotite and sodalite increase in quantity. Such apophyses are developed mainly in the hanging wall of large veins, where they locally form stockwork-like bodies up to 80 m long and 30 m wide.
The corundum crystals have characteristic curved faces and barrel-like, short-prismatic forms. The sizes range from 0.3 to 2 cm in length and from 0.2 to 1.5 cm in diameter. The colour is cornflower-blue and deep blue. Transparent varieties are very rare.
Ref: E.Ya. Kievlenko (2003) Geology of gems, p. 48-49
Gemstones from Kola Peninsula, Murmansk Oblast, Russia by Colour
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