Corundum

Real mineral · uncommon · Al₂O₃

Aluminium oxide, the second hardest natural mineral after diamond. Pure corundum is colourless; chromium produces ruby, while iron and titanium together produce blue sapphire. Used industrially as an abrasive.

Identity

Formula

Al₂O₃

Chemical class

oxide

Mineral group

Ruby, Sapphire

Also known as

Emery (granular variety), Ruby (red gem variety), Sapphire (other gem colors)

Crystallography

Crystal system

trigonal

Habits

tabular, prismatic, rhombohedral, pyramidal

Fracture

conchoidal

Physical properties

Mohs hardness

9

Density

4.02 g/cm³

Luster

vitreous, adamantine

Colors

gray, brown, red, blue, yellow, colorless

Streak

white

Transparency

transparent, translucent, opaque

Geology

Origins

metamorphic, igneous

Host rocks

marble, schist, gneiss, syenite

Associated minerals

Spinel, Chlorite, Garnet, Tourmaline

Geography

worldwide

Classification

Rarity

uncommon

Gem quality

Yes

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