mineral

Corundum (Sapphire variant)

Aluminum Oxide (Al2O3)

Hardness: 9 on the Mohs scale. Color: Greyish-blue to dark blue. Luster: Adamantine to vitreous. Crystal structure: Hexagonal/Trigonal (note the triangular/hexagonal cross-section and visible growth striations). Specific gravity: 3.95 - 4.10.

Hardness: 9 on the Mohs scale
Color: Greyish-blue to dark blue
Luster: Adamantine to vitreous

Identified June 30, 2026More mineral →

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Physical properties

Formation & geological history

Formed in aluminum-rich and silica-poor environments. Commonly found in metamorphic rocks like marble or gneiss, and as a primary mineral in igneous rocks like syenite. It is often recovered from alluvial placer deposits.

Uses & applications

Industrial use as an abrasive due to extreme hardness. Gem-quality specimens are used in jewelry. In this rough form, it is primarily a collector's specimen or lapidary material.

Geological facts

Corundum is the second hardest natural mineral after diamond. The blue variety is known as sapphire, while the red variety is ruby. The visible lines on the surface are 'growth striations' typical of hexagonal crystals.

Field identification & locations

Identify in the field by its hexagonal crystal habit, exceptional weight (high density), and the fact that it can scratch almost any other mineral. Look for it in river gravels near metamorphic terrains.