mineral

Corundum

Corundum (Aluminium oxide, Al2O3)

Hardness: 9 on Mohs scale. Color: Gray, brown, or dull black (non-gem variety). Luster: Adamantine to vitreous. Crystal structure: Trigonal (often hexagonal barrels). Cleavage: None, but exhibits rhombohedral parting. Specific gravity: 3.98–4.10.

Hardness: 9 on Mohs scale
Color: Gray, brown, or dull black (non-gem variety)
Luster: Adamantine to vitreous

Identified April 8, 2026More mineral →

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Physical properties

Formation & geological history

Formed as a primary mineral in igneous rocks (syenite, pegmatite) or in metamorphic rocks such as mica schist, gneiss, and crystalline limestones. The specimen suggests a metamorphic origin from an aluminum-rich environment.

Uses & applications

Abrasives (emery), refractory materials, and industrial cutting tools. Gem-quality varieties are Rubies and Sapphires. Large opaque specimens are used for mineral collecting and teaching.

Geological facts

Corundum is the second hardest natural mineral known to science, surpassed only by diamond. The label in the image suggests it may be from the classic Bedford, New York pegmatite or metamorphic locality.

Field identification & locations

Identify by its extreme hardness (will scratch almost any other mineral), high density, and characteristic hexagonal barrel-shaped crystal habit. Often found in aluminum-rich metamorphic zones.