mineral

Sapphire in a ring

Corundum (Al2O3)

Hardness: 9 on the Mohs scale; Color: Deep blue to near-black; Luster: Vitreous; Crystal Structure: Trigonal/Hexagonal; Cleavage: None; Specific Gravity: 3.95-4.03

Hardness: 9 on the Mohs scale
Color: Deep blue to near-black
Luster: Vitreous

Identified June 17, 2026More mineral →

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Physical properties

Hardness: 9 on the Mohs scale; Color: Deep blue to near-black; Luster: Vitreous; Crystal Structure: Trigonal/Hexagonal; Cleavage: None; Specific Gravity: 3.95-4.03

Formation & geological history

Formed in aluminum-rich igneous and metamorphic environments, specifically pegmatites, basalt, or marble. Most high-quality sapphires are millions of years old.

Uses & applications

Primary use is in high-end jewelry; industrial-grade corundum is used for abrasives, watch crystals, and high-pressure windows.

Geological facts

Sapphire is a variety of the mineral corundum. While it is famous for blue, it can occur in every color except red (which is classified as ruby).

Field identification & locations

Identification in the field usually involves its extreme hardness (cannot be scratched by steel or quartz) and high density; common in alluvial deposits in Sri Lanka, Madagascar, and Thailand.