gemstone
Corundum (Sapphire / Raw Sapphire)
Aluminum Oxide (Al2O3)
Mohs hardness 9, vitreous to adamantine luster, trigonal crystal system, bluish-grey color, high specific gravity (3.98-4.10), conchoidal fracture
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Physical properties
Mohs hardness 9, vitreous to adamantine luster, trigonal crystal system, bluish-grey color, high specific gravity (3.98-4.10), conchoidal fracture
Formation & geological history
Forms in silica-poor igneous rocks (such as syenites and pegmatites) and aluminum-rich metamorphic rocks (such as schists and gneisses), as well as alluvial placer deposits
Uses & applications
Used primarily as gemstone material for jewelry and as an industrial abrasive due to its extreme hardness
Geological facts
Corundum is the second hardest natural substance known after diamond. Blue and non-red gemstone-quality varieties are known as sapphire, while red varieties are classified as ruby.
Field identification & locations
Identified by its high density/weight for its size, resistance to scratching (scratches glass and quartz easily), and classic hexagonal crystal habit or vitreous fracture faces. Commonly found in Sri Lanka, Madagascar, Myanmar, Montana (USA), and Australia.
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