Most people associate the Pearl with the month of June. There are actually three gemstones recognized as the birthstone of June, Pearl, Alexandrite, & the Moonstone.
The Moonstone "was given its name by the Roman natural historian Pliny, who wrote that moonstone's appearance altered with the phases of the moon — a belief that held until well after the sixteenth century. A phenomenal gemstone, moonstones show a floating play of light (called adularescence) and sometimes show either a multirayed star or a cat's eye. Considered a sacred stone in India, moonstones often are displayed on a background of yellow (a sacred color) and are believed to encapsulate within the stone a spirit whose purpose is to bring good fortune. Part of the family of minerals called feldspar, moonstone occurs in many igneous and metamorphic rocks and comes in a variety of colors such as green, blue, peach, and champagne. The most prized moonstones are from Sri Lanka; India, Australia, the United States, Mayanmar, and Madagascar are also sources." http://www.americangemsociety.org/june-birthstones