mineral

Rough Star Sapphire (Corundum)

Al2O3 (Aluminum Oxide)

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.

Hardness: 9 on the Mohs scale
Color: Deep blue with potential asterism
Luster: Adamantine to vitreous

Identified April 14, 2026More mineral →

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Physical properties

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.

Other mineral specimens

See all

Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Schist

Metamorphic

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral