Corundum
Mineral Al2O3
About it
Six-sided barrels, spindles, and tapering prisms that scratch everything except diamond, heavy for a pale mineral. Its gem forms are ruby and sapphire; crystals come from Sri Lanka, Myanmar, Madagascar, and North Carolina.
- Type
- Mineral
- Group
- Oxide, hematite group
- Formula
- Al2O3
- Hardness (Mohs)
- 9
- Specific gravity
- 3.98–4.1
- Streak
- white
- Lustre
- adamantine (diamond-like), vitreous (glassy)
- Crystal system
- Trigonal
- Colours
- grey, brown, blue, red, pink, yellow, violet, colourless
- Transparency
- transparent, translucent, opaque
- Cleavage and fracture
- none; basal and rhombohedral parting, conchoidal to uneven fracture
- Under UV light
- red and pink stones glow red under longwave UV
Typical values: real specimens vary. For more, see mindat.org.