Corundum (from Tamil kurundam குருந்தம் or kuruvindam குருவிந்தம்) is a crystalline form of aluminium oxide and one of the rock-forming minerals. It is naturally clear, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. A pinkish-orange sapphire is called padparadscha.

Due to corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral, leaving behind a streak of white on the other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with a lower average Mohs hardness near 8.0.

In addition to its hardness, corundum is unusual for its high 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. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands.

Corundum for abrasives is mined in Zimbabwe, Russia, and India. Historically it was mined from deposits associated with dunites in North Carolina and from a nepheline syenite in Craigmont, Ontario. Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York. Abrasive corundum is synthetically manufactured from bauxite.

Synthetic corundum

In 1837 Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a little chromium as a pigment. In 1847 Edelman made white sapphire by fusing alumina in boric acid. In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF₂ and Al₂O₃ with a little chromium at temperatures above 2000°C. In 1903 Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.

The Verneuil process allows the production of flawless single-crystal sapphire, rubies and other corundum gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years. Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant watch crystals and windows for optical equipment, spacecraft as well as for lasers.