mineral

Corundum (variety Ruby/Sapphire)

Aluminum Oxide (Al2O3)

Hardness: 9 on Mohs scale. Color: Pinkish-red (hexagonal growth lines visible). Luster: Adamantine to vitreous. Crystal structure: Trigonal/Hexagonal. Cleavage: None (has rhombohedral parting). Specific gravity: 3.95-4.10.

Hardness: 9 on Mohs scale
Color: Pinkish-red (hexagonal growth lines visible)
Luster: Adamantine to vitreous

Identified June 11, 2026More mineral →

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Physical properties

Formation & geological history

Formed in silica-poor environments. Primarily occurs in metamorphic rocks like marble or gneiss, and as a primary mineral in igneous rocks like syenite. Often found in alluvial placer deposits.

Uses & applications

Non-gem quality is used as an industrial abrasive (emery). Gem-quality specimens are used in high-end jewelry. Synthetic versions are used in lasers and watch bearings.

Geological facts

Corundum is the second hardest natural mineral after diamond. The presence of chromium creates the red color of rubies, while titanium and iron create blue sapphires. This specimen shows the characteristic hexagonal barrel shape common in raw corundum crystals.

Field identification & locations

Identify by its extreme hardness (will scratch almost any other mineral) and the presence of hexagonal cross-sections or 'silk' inclusions. Often found in riverbeds or mountain metamorphic complexes in regions like Madagascar, Myanmar, or North Carolina.

Other mineral specimens

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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