Corundum

Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. The unit cell contains six formula units. The toughness of corundum is sensitive to surface roughness and crystallographic orientation. It may be 6–7 MPa·m for synthetic crystals, and around 4 MPa·m for natural. In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the gaps between the octahedra are occupied by aluminium ions.

Though corundum is collected as a precious gem, its hardness level also makes it a useful abrasive. It is used in grinding wheels, sandpaper, polishing metal, and grinding optical glass. It is also sometimes used in refractories due to its high melting point.

Some cut corundums form a unique "star" property with three crossbars and six rays, which determined its historical European use as a talisman to guide travelers. Additionally, it was used to protect those who possessed it from illness and bad omens such as the Evil Eye and devils. Corundum with such asterism was referred to as the "Star of Destiny" by Christians, representing Destiny, Faith and Hope.

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. The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in), and weighed 152 kg (335 lb). The record has since been surpassed by certain synthetic boules. Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US, and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite. Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China.

The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).