Corundum (Ruby/Sapphire crystal)

Physical Properties

Hardness: 9 on Mohs scale. Color: Purplish-red to greyish-pink. Luster: Adamantine to vitreous. Crystal structure: Hexagonal/Trigonal (often barrel-shaped). Cleavage: None, but exhibits parting. Specific gravity: 3.95–4.10.

Formation & Geological History

Formed in silica-poor igneous rocks like syenites and metamorphic rocks such as mica schists, gneisses, and crystalline limestones. Many are found as pebbles in alluvial deposits due to high weather resistance.

Uses & Applications

Industrial Grade: Used as abrasives, grinding wheels, and sandpaper. Gem Grade: Rubies and Sapphires are highly valued in jewelry. Scientific: Used in laser technology and high-pressure windows.

Geological Facts

Corundum is the second hardest natural mineral after diamond. A rubys red color comes from chromium impurities, while blue sapphire comes from iron and titanium. This specimen appears to be 'stark' or opaque corundum, often referred to as common corundum or low-grade ruby.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by steel or quartz) and its distinctive hexagonal crystal habit. Common locations include Myanmar, Sri Lanka, Madagascar, and Tanzania.

Identified on: 4/25/2026

Mode: Standard