Corindo

is a crystalline form of aluminium oxide (Al₂O₃) with traces of iron, titanium and chromium. It is one of the naturally clear transparent materials, but can have different colors when impurities are present.


Transparent specimens are used as gems, called ruby if red and padmaraga if pink-orange. All other colors are called sapphire. Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral. In addition to its hardness, corundum is unusual for its density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low atomic mass elements aluminium and oxygen

Corundum occurs as a mineral in mica schist, gneiss, and some marbles in Metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites.  It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering.