SiO2·nH2OAmorphous
Opal
SiO2·nH2O
MineraloidsChemical FormulaSiO2·nH2O
Crystal SystemAmorphous
Mohs Hardness6
Specific Gravity2.1
ColorAll colors, play of color
LusterVitreous to resinous
StreakWhite
CleavageNone
FractureConchoidal
Occurrence
Formed from silica-rich solutions. Precious opal shows play of color. Australia produces 95%.