/oh"peuhl/ , n.
1. a mineral, an amorphous form of silica, SiO 2 with some water of hydration, found in many varieties and colors, including a form that is milky white.
2. an iridescent variety of this that is used as a gem.
3. a gem of this.
[ 1350-1400; ME opalus opállios opal, gem; prob. from a source akin to Skt upala precious stone ]