mineral

Rough Corundum (Sapphire/Ruby mixture)

Corundum (Aluminum Oxide, Al2O3)

Hardness: 9 on Mohs scale. Color: Grayish, pinkish, and lavender tones. Luster: Adamantine to vitreous. Crystal structure: Hexagonal (Trigonal). Cleavage: None (has parting). Specific gravity: 3.98-4.06.

Hardness: 9 on Mohs scale
Color: Grayish, pinkish, and lavender tones
Luster: Adamantine to vitreous

Identified April 12, 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 like marble and schist. These specimens appear to be water-worn alluvial pebbles often found in river beds.

Uses & applications

Industrial abrasives, watch bearings, and gemstones. If high quality, they are faceted into jewelry; if lower grade, they are used for carving or collection.

Geological facts

Corundum is the second hardest natural mineral known to man. Colored varieties are called sapphires, except for the red variety which is strictly called ruby.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by steel or quartz) and high specific gravity (feels heavy for its size). Commonly found in Sri Lanka, Madagascar, and Montana, USA.

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