From The Gemology Project
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