Moonstone is a combination of two types of feldspar rocks with a translucent layered quality. Light passing through those layers creates the phenomenon with the iridescent glow. Ancient Romans use to believe that the stone was solidified moonbeams.
The rarest therefore the most valuable is the Deep blue color. Although in recent years there has been an upswing in desire for all the colors. This being said, just know, this a stone with rapidly growing value in all colors.