This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it. The multicolored flashes of light that Opal emits gives it a truly beautiful and valuable look. Color is white, colorless, pale yellow, pale red, gray or black when impurities are common. Luster is vitreous to pearly.
Crystals are transparent to translucent.
Crystal System does not apply because opal is amorphous. Habits include massive, cavity-fillings such as in fractures and geodes, nodular or as a replacement of other minerals and wood. Cleavage is absent. Fracture is conchoidal. Streak is white.