Rings around the Moon are produced by ice crystals in the air at high altitudes. These lunar halos, like their solar equivalents, can be very spectacular with rainbow-like colors, offset arcs accompanying the main ring, and 'moon dogs' or mock moons. My favorite reference book for unusual phenomena like this is 'Handbook of Unusual Natural Phenomena' by William Corliss, published by Doubleday Anchor Books in 1983.