Named for its sheen, moonstone is the most well-known gemstone of the feldspar group. Beneath the soft shimmering surface of moonstone surges a deeper, lovelier beauty: a white "moon" that rolls over the stone when it rotates. The scientific term for the kind of shimmer--a moving floating light or sheen--that gives this stone its magical look is schiller or adularescence. The Romans thought moonstone was actually formed out of solidified moonlight. However, the shimmer comes from two types of feldspar growing into or weaving through each other as the crystal forms.