Rough Star Sapphire (Corundum)

Physical Properties

Hardness: 9 on the Mohs scale. Color: Deep blue with potential asterism. Luster: Adamantine to vitreous. Crystal Structure: Hexagonal (trigonal). Cleavage: None (has parting). Specific Gravity: 3.95–4.03.

Formation & Geological History

Formed in metamorphic rocks such as mica schist or gneiss, and in certain igneous rocks like syenite. Often found in secondary alluvial (placer) deposits after weathering out of the host rock over millions of years.

Uses & Applications

Gemstone used in jewelry (usually cabochon cut to display the star), industrial abrasives, and high-durability watch crystals or scientific lenses.

Geological Facts

The star effect, called asterism, is caused by light reflecting off microscopic needle-like inclusions of the mineral rutile (silk) oriented in three directions.

Field Identification & Locations

Identify in the field by its high density (feels heavy) and extreme hardness (will scratch almost anything except diamond). Common locations include Sri Lanka, Myanmar, Thailand, and Madagascar.

Identified on: 4/14/2026

Mode: Standard