Precious opal is hydrated silica, about SiO2·nH2O. It is a mineraloid, not a crystalline mineral, so it has no crystal system. The play-of-color comes from diffraction of light by orderly stacks of silica spheres, not from a pigment.
Hardness is about 5.5 to 6.5. Opal can craze if it dries too fast because it contains water. Common opal is the same material without play-of-color and is not the subject of this page. Fire opal is a warm body-color variety and may or may not show play-of-color.