Al2O3Trigonal
Corundum
Al2O3
OxidesChemical FormulaAl2O3
Crystal SystemTrigonal
Mohs Hardness9
Specific Gravity4.02
ColorColorless, blue, red, yellow, brown
LusterAdamantine to vitreous
StreakWhite
CleavageNone
FractureConchoidal to uneven
Occurrence
Found in metamorphic and igneous rocks. Ruby (red) and sapphire (blue) are gem varieties. Second hardest mineral.