Definition of: opal

(ō′pəl) noun
An amorphous, variously colored, hydrous silica, softer and less dense than quartz. The precious (or noble) opal presents a peculiar play of delicate colors, and is valued as a gemstone. The fire (or flame) opal shows its colors disposed in streaks: often called girasol. [<L opalus <Gk. opallios <Skt. upala a precious stone]