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]

The Collaborative International Dictionary of English v.0.48:

Cacholong \Cach"o*long\, n. [F. cacholong, said to be from Cach,
   the name of a river in Bucharia + cholon, a Calmuck word for
   stone; or fr. a Calmuck word meaning "beautiful stone"]
   (Min.)
   An opaque or milk-white chalcedony, a variety of quartz;
   also, a similar variety of opal.
   [1913 Webster]