opal

A mineral of hydrated silica.

A gemstone made of this mineral, noted for its rich iridescence.

A mineral consisting of silica like quartz, but in a different condition, having a lower specific gravity and hardness and being without crystalline structure: it usually contains some water, mostly from 3 to 9 per cent..

A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity.

A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO2·nH2O.