mineral

Sapphire (Rough Corundum)

Corundum (Aluminum Oxide, Al2O3)

Hardness: 9 on Mohs scale. Color: Greyish-blue, translucent. Luster: Adamantine to vitreous. Crystal structure: Trigonal/Hexagonal. Cleavage: None (has parting). Specific Gravity: 3.95-4.10.

Hardness: 9 on Mohs scale
Color: Greyish-blue, translucent
Luster: Adamantine to vitreous

Identified June 6, 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 Mohs scale. Color: Greyish-blue, translucent. Luster: Adamantine to vitreous. Crystal structure: Trigonal/Hexagonal. Cleavage: None (has parting). Specific Gravity: 3.95-4.10.

Formation & geological history

Formed in silica-poor igneous or metamorphic rocks like marble, basalt, or pegmatites under high pressure and temperature. Often found as alluvial deposits in riverbeds.

Uses & applications

Gemstones for jewelry, industrial abrasives, and high-durability windows or watch crystals due to its extreme hardness.

Geological facts

Corundum is the second hardest natural mineral after diamond. When it occurs in any color other than red (ruby), it is classified as a sapphire.

Field identification & locations

Identified by its high density and inability to be scratched by steel or quartz. Common locations include Sri Lanka, Madagascar, Montana (USA), and Australia. Look for hexagonal barrel-shaped crystal habits.

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

Biotite Schist

Biotite-rich Schist [K(Mg,Fe)3AlSi3O10(F,OH)2]

metamorphic