[1] : The Collaborative International Dictionary of English v.0.48

opal \o"pal\, n. l. opalus: cf. gr. ?, skr. upala a rock,
   stone, precious stone: cf. f. opale. min.
   a mineral consisting, like quartz, of silica, but inferior to
   quartz in hardness and specific gravity.
   1913 webster

   note: the precious opal presents a peculiar play of colors
         of delicate tints, and is highly esteemed as a gem. one
         kind, with a varied play of color in a reddish ground,
         is called the harlequin opal. the fire opal has
         colors like the red and yellow of flame. common opal
         has a milky appearance. menilite is a brown impure
         variety, occurring in concretions at menilmontant, near
         paris. other varieties are cacholong, girasol,
         hyalite, and geyserite.
         1913 webster
see also:
precious opal harlequin opal fire opal common opal menilite cacholong 
girasol hyalite geyserite 

Opal >> O Words
Opal, definition of term: Opal
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Precious >> P Words
Precious, definition of term: Precious
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Precious Stones >> P Words
Precious Stones, definition of term: Precious Stones
precious+stones_pag1.html

Precious Metals >> P Words
Precious Metals, definition of term: Precious Metals
precious+metals_pag1.html

Precious Stone >> P Words
Precious Stone, definition of term: Precious Stone
precious+stone_pag1.html