Moonstone is a variety of the feldspar-group mineral orthoclase. During formation, orthoclase and albite separate into alternating layers. When light falls between these thin layers it is scattered, producing the phenomenon; adularescence. This is the light that appears to billow across a gem, giving its surface a glowing appearance. A captivating aspect of adularescence is its appearance of motion. The misty light seems to roll across the gem's surface as you change the viewing angle.
Other feldspar minerals can also show adularescence. Found mainly in Labrador, Canada is the labradorite, which has a multicoloured adularescence over a light body colour. It's known in the trade as rainbow moonstone, even though it is not a variety of orthoclase. Sanidine is another. To be called moonstone, a mineral's actual identity is not as important as the beauty of its adularescence.