The Romans thought that moonstone was formed out of moonlight. Moonstone is a variety of feldspar and the shimmer, which is called schiller or adularescence, is caused by the intergrowth of two different types of feldspar, with different refractive indexes. In Europe, moonstone is considered the birthstone for June, although in the United States it shares that distinction with alexandrite and pearl.
Moonstones come in a variety of colors.
The body color can range from colorless to gray, brown, yellow, green, or pink. The clarity ranges from transparent to translucent. The best moonstone has a blue sheen, perfect clarity, and a colorless body color. Sometimes moonstone will have an eye as well as a sheen. Another related feldspar variety is known as rainbow moonstone.