It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire.

[8] Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colours to trace impurities.

[11][12] A 2019 textbook by Andrew Ruys contains a detailed timeline on the history of aluminium oxide from ancient times to the 21st century.

In its most commonly occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.

In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space group of R3c (number 167 in the International Tables).

[18] Molten aluminium oxide near the melting temperature is roughly 2/3 tetrahedral (i.e. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) octahedral Al-O present.

[19] Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing.

[23] Firstly, Bauxite is mixed with additives like limestone and soda ash, then heating the mixture at high temperatures (1200 °C to 1500 °C) to form sodium aluminate and calcium silicate.

[26] Known as alpha alumina in materials science, and as alundum (in fused form) or aloxite[27] in mining and ceramic communities, aluminium oxide finds wide use.

Large tonnages of aluminium hydroxide, from which alumina is derived, are used in the manufacture of zeolites, coating titania pigments, and as a fire retardant/smoke suppressant.

Aluminium oxide is a common ingredient in sunscreen[29] and is often also present in cosmetics such as blush, lipstick, and nail polish.

In its largest scale application, aluminium oxide is the catalyst in the Claus process for converting hydrogen sulfide waste gases into elemental sulfur in refineries.

As the powdery abrasive mineral aloxite, it is a major component, along with silica, of the cue tip "chalk" used in billiards.

[33] Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints.

[34] In this case, aluminium oxide is used to coat the surfaces of medical implants to give biocompatibility and corrosion resistance.

Some body armors utilize alumina ceramic plates, usually in combination with aramid or UHMWPE backing to achieve effectiveness against most rifle threats.

Alumina ceramic armor is readily available to most civilians in jurisdictions where it is legal, but is not considered military grade.

[citation needed] Alumina is used to manufacture tiles which are attached inside pulverized fuel lines and flue gas ducting on coal fired power stations to protect high wear areas.

[41][42] For its application as an electrical insulator in integrated circuits, where the conformal growth of a thin film is a prerequisite and the preferred growth mode is atomic layer deposition, Al2O3 films can be prepared by the chemical exchange between trimethylaluminium (Al(CH3)3) and H2O:[43] H2O in the above reaction can be replaced by ozone (O3) as the active oxidant and the following reaction then takes place:[44][45] The Al2O3 films prepared using O3 show 10–100 times lower leakage current density compared with those prepared by H2O.

[50] Using a plasma spray process and mixed with titania, it is coated onto the braking surface of some bicycle rims to provide abrasion and wear resistance.

[citation needed] Most ceramic eyes on fishing rods are circular rings made from aluminium oxide.

[citation needed] In its finest powdered (white) form, called Diamantine, aluminium oxide is used as a superior polishing abrasive in watchmaking and clockmaking.