Opal
Chemical Formula	SiO2·nH2O
Species	Silicates
Mohs Scale	5-6
Specific Gravity	2.15
Color	Colorless, white, yellow, red, orange, green, brown, black, blue
Streak	White
Luster	Vitreous, Waxy, Greasy, Dull
Refractive Index	1.450 (+.020, -.080) Mexican opal may read as low as 1.37, but typically reads 1.42–1.43[1]
Diaphaneity	Transparent, Opaque
Cleavage	None Observed
Fracture	Irregular/Uneven, Splintery, Conchoidal
Crystal Habit：Irregular veins, in masses, in nodules
Geological Setting：Altering volcanic tuffs, basalts