Moonstones have phantoms of flash! As you turn the stone the flash of color, the mysterious sheen, moves around the gem, appearing and disappearing. Moonstones are orthoclase feldspar with a sheen called adularescence.
The most elite of the moonstones have blue flash or a rainbow of colors. The body color of these moonstones are colorless and semi-transparent. Tiny platelets which are too small to see create the adularescence, reflecting back shimmering color. Some colorless moonstones have a white flash. Blue flash and rainbow moonstones have recently become very popular. Once considered plentiful, they are now rather scarce due to increased demand and prices have gone up.