Corundum

Corundum is a crystalline form of aluminum oxide (Al₂O₃) that usually contains trace amounts of iron, titanium, vanadium, and chromium. As a rock-forming mineral, it is naturally transparent but takes on different colors depending on which transition metal impurities are present in its crystal structure. Its two main gem varieties are ruby and sapphire. Ruby gets its red color from chromium, while sapphire can appear in many colors based on the specific transition metal involved; a rare pink-orange type is known as padparadscha sapphire. The name "corundum" comes from the Tamil-Dravidian word *kurundam* (meaning ruby-sapphire), which also appears in Sanskrit as *kuruvinda*.

Thanks to its hardness—pure corundum is rated 9.0 on the Mohs scale—it can scratch nearly all other minerals. Emery, a black granular form of corundum with no gem value, is mixed with magnetite, hematite, or hercynite and is widely used as an abrasive on sandpaper and large tools for machining metals, plastics, and wood. Corundum also has a density of 4.02 g/cm³ (251 lb/cu ft), which is unusually high for a transparent mineral made of low-atomic-mass elements like aluminum and oxygen.

**Geology and occurrence**

Corundum forms in metamorphic settings such as mica schist, gneiss, and some marbles. It also appears in low-silica igneous rocks like syenite and nepheline syenite intrusions, as well as in masses next to ultramafic intrusives, alongside lamprophyre dikes, and as large crystals in pegmatites. Because it is hard and resists weathering, it often turns up as a detrital mineral in stream and beach sands. The largest documented natural single crystal of corundum measured about 65 cm × 40 cm × 40 cm and weighed 152 kg (335 lb), though that record has since been beaten by certain synthetic boules.

Abrasive corundum is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically, it was extracted from deposits linked to dunites in North Carolina, USA, and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, USA. Synthetic abrasive corundum is manufactured from bauxite. Four corundum axes dating to 2500 BC, from China’s Liangzhu culture and Sanxingcun culture (the latter in Jintan District), have been discovered.

**Synthetic corundum**

In 1837, Marc Antoine Gaudin created the first synthetic rubies by heating alumina with a small amount of chromium as a colorant. In 1847, J. J. Ebelmen made white synthetic sapphires by reacting alumina in boric acid. In 1877, Frenic and Freil produced crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF₂ and Al₂O₃ with small amounts of chromium at temperatures above 2,000 °C (3,630 °F). In 1903, Verneuil announced a commercial-scale flame fusion process for synthetic rubies.

This flame fusion method allows flawless single-crystal sapphire and ruby gems to be produced much larger than those found in nature. Gem-quality synthetic corundum can also be grown through flux-growth and hydrothermal synthesis. Because the synthesis methods are straightforward, large quantities of these crystals are now available on the market at a fraction of the cost of natural stones. Synthetic corundum has a lower environmental impact than natural corundum, as it avoids destructive mining and conserves resources, but its production is energy-intensive—contributing to carbon emissions if fossil fuels are used—and involves chemicals that can pose risks.

Beyond ornamental uses, synthetic corundum is employed in mechanical parts (tubes, rods, bearings, and machined components), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (due to its transparency from ultraviolet to infrared), and laser components. For instance, the KAGRA gravitational wave detector’s main mirrors are 23 kg (50 lb) sapphires, and Advanced LIGO considered 40 kg (88 lb) sapphire mirrors. Corundum is also used in ceramic armor because of its high hardness.

**Structure and physical properties**

Corundum crystallizes with trigonal symmetry in the space group R3c, with lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. Its unit cell contains six formula units. The toughness of corundum depends on surface roughness and crystallographic orientation, ranging from 6–7 MPa·m¹/² for synthetic crystals to about 4 MPa·m¹/² for natural ones. In its lattice, oxygen atoms form a slightly distorted hexagonal close packing, with two-thirds of the octahedral sites occupied by aluminum ions. The absence of aluminum from one of the three sites breaks the hexagonal close packing symmetry, reducing the space group to R3c and the crystal class to trigonal, so the structure is sometimes described as pseudohexagonal.

Young’s modulus for corundum (sapphire) has been reported between 300 and 500 GPa, with a commonly cited value of 345 GPa. It is temperature dependent: in the [0001] direction, it is 435 GPa at 323 K and 386 GPa at 1,273 K. The shear modulus is 145 GPa, and the bulk modulus is 240 GPa. Single-crystal corundum fibers, which have potential in high-temperature composites, show a Young’s modulus that varies strongly with crystallographic orientation along the fiber axis, reaching a maximum of 461 GPa.