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opal (n.)

"mineral like quartz but without crystalline structure," 1590s, from French opalle (16c.) and directly from Late Latin opalus (Pliny), supposedly from Greek opallios, which is possibly ultimately from Sanskrit upala-s "gem, precious stone." Used in Middle English in Latin form (late 14c.). Related: Opaline.

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Definitions of opal from WordNet

opal (n.)
a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones;
From wordnet.princeton.edu