Opal
/ˈoʊpəl/
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Definition
(noun) A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO₂·nH₂O.
Sentence Examples
Is that an opal in your ring?
An opal is a form of silicon.
CEFR Practice Quiz
The ring was set with a beautiful ____ that shimmered with blue and green hues in the sunlight.