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]