mineral

Rough Sapphire

Corundum (Al2O3)

Hardness: 9 on Mohs scale. Color: Deep blue with greenish undertones. Luster: Vitreous (glassy). Crystal Structure: Hexagonal/Trigonal. Cleavage: None, but shows parting. Specific gravity: 3.95-4.03.

Hardness: 9 on Mohs scale
Color: Deep blue with greenish undertones
Luster: Vitreous (glassy)

Identified May 7, 2026More mineral →

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Physical properties

Hardness: 9 on Mohs scale. Color: Deep blue with greenish undertones. Luster: Vitreous (glassy). Crystal Structure: Hexagonal/Trigonal. Cleavage: None, but shows parting. Specific gravity: 3.95-4.03.

Formation & geological history

Formed in aluminum-rich metamorphic rocks (such as schist or gneiss) or igneous rocks (alkaline basalts or syenites). Often found in alluvial deposits. Geological age varies greatly by location.

Uses & applications

Primarily used in jewelry as a gemstone. Industrial grade corundum is used as an abrasive or in scratch-resistant glass (smartwatch screens, laboratory windows).

Geological facts

Sapphire is the birthstone for September and is the second hardest natural substance on Earth after diamond. While usually blue, sapphires can occur in every color except red (which is ruby).

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by steel or quartz) and high density. Commonly found in gem gravels in Sri Lanka, Madagascar, and Montana, USA.