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. Some opal fluoresces and it can be very sensitive to impacts and low temperatures.