The Collaborative International Dictionary of English v.0.48:

Hydrophane \Hy"dro*phane\, n. [Hydro-, 1 + Gr. ? to show,
   appear: cf. F. hydrophane.] (Min.)
   A semitranslucent variety of opal that becomes translucent or
   transparent on immersion in water.
   [1913 Webster]