Image: James St. John (CC BY 2.0)
cubiccommonOxides
Sapphire
Al₂O₃
A large, dark, blocky crystal of sapphire, approximately 7.5 centimeters across, exhibiting a somewhat hexagonal or tabular habit.
Sapphire Physical Properties
Mohs Hardness
9Crystal System
cubicLuster
vitreousStreak Color
WhiteCleavage
None (exhibits basal and rhombohedral parting)Fracture
Conchoidal to unevenSpecific Gravity
3.95Colors
blue, pink, yellow, green, purple, orange, black, colorlessTransparency
Transparent to translucentType Locality
Pennsylvania, USAMineral Group
OxidesUses
jewelry, abrasives, watch bearings, scientific instrumentsAssociated Minerals
feldspar, mica, garnet, kyanite, sillimaniteRarity
commonGeological Context
Sapphire forms in aluminum-rich, silica-poor igneous rocks (such as syenites and pegmatites) and metamorphic rocks (like schists and gneisses), or as detrital grains in placer deposits.
