This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. A mineral of hydrated silica.

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

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

n. , (genetics), (biochemistry) A colloquial name used in molecular biology referring to a particular stop codon sequence, "UGA."

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

n. 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. ; ; ; ; ; ; ;

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

"Although opal is not a conclusive indicator of a wetter Mars, it does suggest some interesting scenarios."

mineral    opaque gem   

opalglass slip    opal glass    opal plate    harlequin opal    ceylonese opal   

fire opal    girasol    black opal    harlequin opal   

Constantinople    Opel   

amethyst    turquoise    lazuli    emerald    onyx    garnet    aquamarine    amber    beryl    coral