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.