The Collaborative International Dictionary of English v.0.48:

Harlequin \Har"le*quin\ (h[aum]r"l[-e]*k[i^]n or -kw[i^]n), n.
   [F. arlequin, formerly written also harlequin (cf. It,
   arlecchino), prob. fr. OF. hierlekin, hellequin, goblin, elf,
   which is prob. of German or Dutch origin; cf. D. hel hell.
   Cf. Hell, Kin.]
   A buffoon, dressed in party-colored clothes, who plays
   tricks, often without speaking, to divert the bystanders or
   an audience; a merry-andrew; originally, a droll rogue of
   Italian comedy. --Percy Smith.
   [1913 Webster]

         As dumb harlequin is exhibited in our theaters.
                                                  --Johnson.
   [1913 Webster]

   Harlequin bat (Zool.), an Indian bat (Scotophilus
      ornatus), curiously variegated with white spots.

   Harlequin beetle (Zool.), a very large South American
      beetle (Acrocinus longimanus) having very long legs and
      antenn[ae]. The elytra are curiously marked with red,
      black, and gray.

   Harlequin cabbage bug. (Zool.) See Calicoback.

   Harlequin caterpillar. (Zool.), the larva of an American
      bombycid moth (Euch[ae]tes egle) which is covered with
      black, white, yellow, and orange tufts of hair.

   Harlequin duck (Zool.), a North American duck
      (Histrionicus histrionicus). The male is dark ash,
      curiously streaked with white.

   Harlequin moth. (Zool.) See Magpie Moth.

   Harlequin opal. See Opal.

   Harlequin snake (Zool.), See harlequin snake in the
      vocabulary.
      [1913 Webster]

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]

WordNet (r) 3.0 (2006):

harlequin opal
    n 1: a reddish opal with small patches of brilliant color