They were very popular with the Romans.
Moonstone reflects light in a irridescent sheen called called adularescence.
Moonstones come in a variety of colors. The body color can range from colorless to white,gray, blue, 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. Another related variety of moonstone is known as rainbow moonstone.