opal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A mineral of hydrated silica.
• noun A gemstone made of this mineral, noted for its rich iridescence.

from The Century Dictionary.

• noun 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. ; ; ; ; ; ; ;

from the GNU version of the Collaborative International Dictionary of English.

• noun (Min.) A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO2·nH2O.
• noun biology , (genetics), (biochemistry) A colloquial name used in molecular biology referring to a particular stop codon sequence, "UGA."

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones