The Collaborative International Dictionary of English v.0.48:

Opalescence \O`pal*es"cence\, n. (Min.)
   A reflection of a milky or pearly light from the interior of
   a mineral, as in the moonstone; the state or quality of being
   opalescent.
   [1913 Webster]

WordNet (r) 3.0 (2006):

opalescence
    n 1: the visual property of something having a milky brightness
         and a play of colors from the surface [syn: opalescence,
         iridescence]