Corundum (Sapphire in Host Rock)

Physical Properties

Hardness: 9 on the Mohs scale; Color: Variable (blue, grey, white); Luster: Adamantine to vitreous; Crystal structure: Trigonal/Hexagonal; Cleavage: None (has parting); Specific gravity: 3.98–4.06

Formation & Geological History

Formed in aluminum-rich igneous rocks (syenites, pegmatites) or metamorphic environments (marble, schists) via high pressure and temperature. Found globally in deposits spanning many geological ages.

Uses & Applications

High-quality crystals are used as gemstones (Sapphires/Rubies). Industrial-grade specimens are used as abrasives due to extreme hardness. Common in mineral collections and watchmaking.

Geological Facts

Corundum is the second hardest natural mineral after diamond. Blue varieties are known as sapphire, while red varieties are rubies. All other colors are referred to as fancy sapphires.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by steel or quartz) and high density. Commonly found in alluvial deposits or within light-colored igneous/metamorphic host rocks.

Identified on: 4/22/2026

Mode: Standard