Two varieties of corundum exhibit asterism: star ruby and star sapphire. The Burma ruby is bright red w/ strong dichroism with silky 6 and 12 ray stars. Thai ruby is red garnet with less dichroism and no silk. It is uniaxial negative. Sapphires show strong dichroism. Green sapphire has slightly higher SG /RI.Corundum occurs in massive amounts and used as abrasive. It is a widespread and common aluminum oxide mineral, from which the gemstones ruby and sapphire, and the abrasive material emery, are made. Common corundum, which includes all dull, opaque varieties, is usually light blue to gray, brown, or black. luster adamantine to vitreous. It forms pyramidal or barrellike crystals (hexagonal system) and granular masses. It occurs in feldspathoid bearing igneous rocks as an accessory mineral, in recrystallized limestone, in placer sands, and in aluminum-rich metamorphic rocks.