Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

opalescence
     n : the visual property of something having a milky brightness
         [syn: {iridescence}]