corundum, inclusions.5, gems

Describes thin diaspore lamellae in blue-grey, barrel-shaped corundum crystals up to 5 cm long from Mato Grosso, Brazil. Under slight mechanical pressure, the crystals split readily along gently curved surfaces lacking a consistent crystallographic orientation. A thin mineral layer exposed on these surfaces has a lower refractive index than corundum and was identified as diaspore. The unusual splitting is attributed to these lamellae, interpreted as products of retrograde metamorphism.