Opal

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Opal
Chemical composition SiO2+.H2O
Crystal system Amorphous
Hardness 5- 6 1/2
Refractive index 1.45 (+.020;-0.080)
Specific gravity 2.15 (+0.07;-0.90)
Lustre Vitreous
Phenomenom Play of Color
Precious Mexican Crystal Opal
Photo courtesy of Rick Martin, Art Cut Gems

Yellow Common Opal
Photo courtesy of Rick Martin, Art Cut Gems

Mintabe Opal Rub
Photo courtesy of Kevin Schwebel, Handmade Enterprises