D defs.my
Entry 1 sense Webster, 1913

Opal

/ōp'-əl/ · O·pal · IPA /ˈoʊpəl/
01 n. A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity.
  1. 1.
    A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity.(Min.)