mineral

Corundum (Sapphire variant)

Aluminium oxide (Al2O3)

AI-generated identification · may be incorrect

Hardness: 9 on Mohs scale; Color: Grayish-brown to dull greenish-blue; Luster: Adamantive to vitreous (can appear greasy when worn); Crystal structure: Trigonal/Hexagonal; Cleavage: None (has parting); Specific gravity: 3.95–4.10.

Hardness: 9 on Mohs scale
Color: Grayish-brown to dull greenish-blue
Luster: Adamantive to vitreous (can appear greasy when worn)

Identified March 15, 2026More mineral →

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Physical properties

Formation & geological history

Formed as a primary mineral in igneous rocks like syenite and pegmatite, or in metamorphic rocks such as mica schist and gneiss. Often found in alluvial deposits (placer deposits) due to its extreme hardness and resistance to weathering.

Uses & applications

Non-gem quality corundum is used heavily as an industrial abrasive. Low-grade specimens are popular for gemstone tumbling practice or as reference mineral specimens for collectors.

Geological facts

Corundum is the second hardest natural mineral known to man, behind only diamond. While red corundum is called ruby, all other colors are technically sapphires. Stones with this specific barrel-like shape are often referred to as 'corundum crystals' or 'sapphire rough'.

Field identification & locations

Identify in the field by its characteristic barrel shape (hexagonal bipyramidal), extreme heaviness for its size, and the fact that it will easily scratch glass, quartz, and topaz. Commonly found in riverbeds in Montana, North Carolina, and Southeast Asia.