catflap.org Online Dictionary Query

  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]