The Collaborative International Dictionary of English v.0.48:

Opalescent \O`pal*es"cent\, a.
   Reflecting a milky or pearly light from the interior; having
   an opaline play of colors.
   [1913 Webster]

WordNet (r) 3.0 (2006):

opalescent
    adj 1: having a play of lustrous rainbow colors; "an iridescent
           oil slick"; "nacreous (or pearlescent) clouds looking
           like mother-of-pearl"; "a milky opalescent (or opaline)
           luster" [syn: iridescent, nacreous, opalescent,
           opaline, pearlescent]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "opalescent":
   burele, chatoyant, creamy, delicate, eggshell, flat, gloss, iridal,
   iridescent, iridial, iridian, irisated, irised, light, lustrous,
   mellow, moire, mother-of-pearl, nacre, nacred, nacreous, nacry,
   opaline, opaloid, pale, pastel, patinaed, pavonian, pavonine,
   pearl-like, pearlish, pearly, quiet, rainbowlike, rainbowy, sad,
   semigloss, simple, sober, soft, soft-colored, soft-hued, softened,
   somber, subdued, subtle, sweet, tender, tortoise-shell