Ball-and-stick representation of the corundum crystal structure of α-Al₂O₃. Aluminium atoms are shown in blue and oxygen atoms in red; each Al atom is octahedra — Photo: Editistic · CC0
Corundum
Also known as: Emery (rock), Ruby & sapphire are gem varieties.
Crystalline aluminium oxide — the second-hardest natural mineral (Mohs 9) whose gem varieties are ruby and sapphire.
Intro adapted from Wikipedia (CC BY-SA).
Corundum is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Al₂O₃.
It crystallizes in the hexagonal crystal system (space group R-3c). It is a member of the Corundum structural group.
Key properties: Mohs hardness 9; specific gravity 3.98–4.1; typically gray or brown or blue or red or pink or yellow or colorless; White streak; Adamantine to vitreous; pearly on partings luster; refractive index 1.765; optically uniaxial.
IMA symbol Crn, grandfathered status — known before the IMA’s founding.
Characteristic of Al-rich, Si-poor geological environments; in syenite and monzonite, and some quartz-free pegmatites; primary or a reaction product in eclogitic xenoliths in kimberlites. In regional or contact.
It is found associated with Andesine, oligoclase, nepheline, scapolite (syenites); spinel, rutile, chondrodite, “hornblende”, phlogopite, calcite (metamorphosed limestones); kyanite.
Probably from the Sanskrit kurivinda, for ruby, through the Tamil kurundam.
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