Rainbow moonstone is technically transparent labradorite (plagioclase feldspar) with a sheen in a variety of iridescent colors. It is a closely related feldspar mineral to actual moonstone which is orthoclase feldspar. Rainbow moonstone is usually colorless and heavily included, especially in larger sizes and exhibits a highly prized blue sheen. Less commonly, a multicolored sheen is present, hence its descriptive name. This beautiful and unusual gem is mostly fashioned into cabochons, but faceted gems may be found in smaller sizes.