Source : Webster's Revised Unabridged Dictionary (1913)

Moonstone \Moon"stone`\, n. (Min.)
   A nearly pellucid variety of feldspar, showing pearly or
   opaline reflections from within. It is used as a gem. The
   best specimens come from Ceylon.

Source : WordNet®

moonstone
     n : a transparent or translucent gemstone with a pearly luster;
         some specimens are orthoclase feldspar and others are
         plagioclase feldspar