Moonstone is a gemstone in the feldspar family. The Romans believed that it was made from solidified rays from the moon. It has a beautiful shimmery look, called adularescence, from which it got its name. The effect is caused by reflections from alternating layers of two minerals called orthoclase and albite.
(In the images, the very bright white dots are simply reflections from my camera! The moonstone effect is the blue-ish shimmery sheen across almost the whole stone.)