mineral

Corundum (Sapphire)

Corundum (Al2O3)

Hardness: 9 on the Mohs scale; Color: Deep blue; 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: Deep blue
Luster: Adamantine to vitreous

Identified June 9, 2026More mineral →

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Physical properties

Hardness: 9 on the Mohs scale; Color: Deep blue; 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 and metamorphic rocks like marble, schist, or syenite; often found in secondary alluvial deposits (river beds) after being weathered out of the primary source.

Uses & applications

Used primarily as a gemstone in jewelry. Non-gem quality industrial corundum is used as an abrasive (sandpaper, grinding wheels) due to its extreme hardness.

Geological facts

Sapphire is the second hardest natural substance on Earth after diamond. While blue is the most famous color, sapphires can occur in almost any color except red (which are Ruby).

Field identification & locations

Identify by its extreme hardness (cannot be scratched by steel or quartz) and its high density (feels heavy for its size). Commonly found in Sri Lanka, Madagascar, and Montana, USA.