gemstone

Raw Sapphire (Corundum)

Aluminum oxide (Al2O3)

Hardness: 9 on the Mohs scale. Color: Grayish-blue, blue. Luster: Vitreous to adamantine. Crystal structure: Trigonal (hexagonal prisms/barrel-shaped crystals). Specific gravity: 3.98-4.10. Cleavage: None (parting present).

Hardness: 9 on the Mohs scale
Color: Grayish-blue, blue
Luster: Vitreous to adamantine

Identified July 22, 2026More gemstone →

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Physical properties

Formation & geological history

Formed through regional or contact metamorphism of silica-poor aluminum-rich rocks, as well as in igneous pegmatites and nepheline syenites. Frequently found in secondary alluvial placer deposits.

Uses & applications

Used primarily in fine jewelry when cut and polished. Industrial uses include abrasives, scratch-resistant optical windows, watch crystals, and specialized electronic substrates.

Geological facts

Sapphire is the non-red gem-quality variety of the mineral corundum (red corundum is ruby). Its extreme hardness makes it second only to diamond among natural minerals.

Field identification & locations

Identified by its high density/weight for its small size, extreme hardness (scratches quartz and topaz easily), barrel shape or hexagonal growth striations, and characteristic glassy-to-adamantine luster on fracture surfaces. Common locations include Sri Lanka, Madagascar, Montana (USA), Myanmar, Australia, and Thailand.