The Collaborative International Dictionary of English v.0.48:

Adularia \Ad`u*la"ri*a\, n. [From Adula, a mountain peak in
   Switzerland, where fine specimens are found.] (Min.)
   A transparent or translucent variety of common feldspar, or
   orthoclase, which often shows pearly opalescent reflections;
   -- called by lapidaries moonstone.
   [1913 Webster]