MOONSTONE

gem-quality feldspar mineral, a mixed sodium and potassium aluminosilicate, (K,Na)AlSi3O8, that shows a silvery or bluish iridescence. Nearly all commercial moonstones come from Dumbara District, Sri Lanka, where they occur in gem gravels and in acid granulites and pegmatites. The term moonstone also has been applied to the plagioclase feldspars peristerite and labradorite, which also exhibit iridescence.