mineral

Corundum (Sapphire in Host Rock)

Corundum (Aluminum Oxide, Al2O3)

Hardness: 9 on the Mohs scale; Color: Variable (blue, grey, white); Luster: Adamantine to vitreous; Crystal structure: Trigonal/Hexagonal; Cleavage: None (has parting); Specific gravity: 3.98–4.06

Hardness: 9 on the Mohs scale
Color: Variable (blue, grey, white)
Luster: Adamantine to vitreous

Identified April 22, 2026More mineral →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 9 on the Mohs scale; Color: Variable (blue, grey, white); Luster: Adamantine to vitreous; Crystal structure: Trigonal/Hexagonal; Cleavage: None (has parting); Specific gravity: 3.98–4.06

Formation & geological history

Formed in aluminum-rich igneous rocks (syenites, pegmatites) or metamorphic environments (marble, schists) via high pressure and temperature. Found globally in deposits spanning many geological ages.

Uses & applications

High-quality crystals are used as gemstones (Sapphires/Rubies). Industrial-grade specimens are used as abrasives due to extreme hardness. Common in mineral collections and watchmaking.

Geological facts

Corundum is the second hardest natural mineral after diamond. Blue varieties are known as sapphire, while red varieties are rubies. All other colors are referred to as fancy sapphires.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by steel or quartz) and high density. Commonly found in alluvial deposits or within light-colored igneous/metamorphic host rocks.

Other mineral specimens

See all

Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral