Pronunciation: (mOOn'stōn"),[key] —n. 1. Also called precious moonstone. a semitransparent or translucent, opalescent, pearly-blue variety of adularia, used as a gem.
2. any of several adularescent feldspars, as certain varieties of albite, labradorite, or oligoclase, used as gems.
3. (not used technically) any milky or girasol stone used as a gem.