|Rainbow Moonstone is actually a colorless labradorite.
It is a potassium feldspar of the orthoclase species (orthoclase feldspar), which in certain directions reflects a bluish, milky light that has been compared with the light of the moon. You will also occasionally see pink or yellow in this light.
This shimmer is called adularescence.
Thin, alternating layers of orthoclase and albite create the light interference that is responsible for it's "shiller" or "floating moon" effect.