Corundum is a crystalline form of aluminium oxide mineral, having traces of chromium, iron and titanium, is a rock-forming mineral. It is among the naturally transparent materials, but impurities can affect its transparency. Transparent crystals are used as gems, the red ones are called ruby while pink-orange variety is called Padparadscha. The rest of the colors are called sapphire, e.g., "green sapphire" for a green crystal.
Derivation of Name
The name "corundum" is derived from the Sanskrit word Kuruvinda or Tamil word Kuruvindam meaning ruby.
The mineral Corundum is found in mineral form in mica schist (metamorphic rock- medium grade showing grains of sheet like near-parallel orientation), gneiss (Rock fashioned by superlative area wise metamorphic processes from available formations) and in marbles existing in metamorphic environment. It is also found in intrusives of Nepheline Syenite and Igneous Syenite of low silica content). Other sources are as masses adjoining ultramafic intrusives, connected by lamprophyre dikes and as big crystals in pegmatites. It normally occurs as a detrital mineral (that has resulted from erosion and weathering of pre-existing rocks), in streams and beach sands due its hardness and resistance to weathering. The biggest recognized single crystal of corundum approximately measured 65×40×40 cm (26×16×16 in), and weighed 152 kg (335 lb). The record has since been exceeded by certain synthetic boules.
Corundum comes in the form of prisms that are hexagonal and barrel-shaped, tapering at both extremities, or as hexagonal plates in thin tubular shape. A lack of a dominant cleavage is evident, besides which conchoidal (fracture in solid minerals resulting in a round, smooth surface) is also absent. With a 9 hardness on the Mohs scale, it is among the toughest commercial gemstones.
Mining of the abrasive material Corundum is from Russia, Zimbabwe, India and Sri Lanka. Traditionally the mining was from the deposits linked with the dunite of North Carolina in the USA and also from deposits of nephelline syenite of Craigmont, in Ontario. Corundum of the emery grade occurs on the Island of Naxos in Greece and places like New York, Peekskill in the USA.
Because of its hardness, corundum can scratch almost all other minerals. It is most often used as an abrasive on sandpaper and tools needed for machining of metals, wood and plastics. The emery paper contains a mixture of corundum and other materials. This mixture is less abrasive, having a Mohs hardness of 8.0 or less. Other than its hardness, corundum possesses a typical density of 4.02 g/cm3, which is very high for a transparent mineral comprising of oxygen and aluminum elements having low atomic mass.
Colorless, brown, gray; pink to pigeon-blood-red, yellow, green, orange, violet; blue to cornflower blue, may be color zoned, asteriated largely gray and brown