The Collaborative International Dictionary of English v.0.48:

Moonstone \Moon"stone`\ (m[=oo]n"st[=o]n`), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

moonstone
    n 1: a transparent or translucent gemstone with a pearly luster;
         some specimens are orthoclase feldspar and others are
         plagioclase feldspar