Introduction: the mineral corundum, which includes the varieties ruby and sapphire, can be formed by both igneous and metamorphic processes. In igneous rocks it is typically found in silica-poor compositions, such as pegmatites or other rocks associated with nepheline syenites (Deer et al., 1992). It may also be found in aluminium-rich nodules in igneous rocks. Metamorphic corundum forms in silica-poor rocks in contact metamorphic zones and in thermally or contact metamorphosed bauxite deposits (Deer et al., 1992).

Occurrence in Wales: corundum has been recorded from altered rhyolitic lavas and rhyolitic tuffs from a borehole near Treffgarne, Pembrokeshire (Brown et al., 1987). Previously Heard (1922) identified prismatic-blue corundum crystals in sandstones and shales of the Pennant Series (Upper Coal Measures) of Carboniferous age exposed to the east of the River Taff, in South Wales.

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