Moonstone is a silicate based feldspar that displays a pearly opalescent schiller. The same effect happens with tigers eye and labradorite. Moonstones have been admired since the Roman Empire and they believed the shine was derived from solidified rays of the moon.
Interesting fact, The moonstone is the state of Florida's state gemstone but it isn't naturally found in Florida. It was designated that in 1970 to commemorate the moon ladings which took off from Kennedy Space Center.