Ruby (Corundum)

Physical Properties

Hardness: 9.0 on Mohs scale; Color: Pinkish-red to blood-red; Luster: Vitreous to sub-adamantine; Crystal Structure: Trigonal/Hexagonal; Cleavage: None (has parting); Specific Gravity: 3.97–4.05

Formation & Geological History

Formed in metamorphic rocks (schist or marble) or igneous rocks (basalt or pegmatite) through high-pressure, high-temperature processes where aluminum is present and silica is absent. Often found in secondary alluvial deposits.

Uses & Applications

Primary use is in high-end jewelry (one of the four precious gemstones). Also used in industrial applications like laser technology, watch bearings, and precision instruments due to its extreme hardness.

Geological Facts

The red color is caused specifically by chromium. Without chromium, the mineral corundum is known as sapphire. The most famous and valuable rubies are historically found in the Mogok Valley of Myanmar (Burma).

Field Identification & Locations

Identify in the field by its extreme hardness (scratches almost everything except diamond), hexagonal crystal habit, and distinct red color. Commonly found in river gravels (placer deposits) or embedded in marble matrix.

Identified on: 4/25/2026

Mode: Standard