1911 Encyclopædia Britannica/Moonstone

MOONSTONE, a variety of felspar, showing in certain directions a bluish opalescence, whence its value as an ornamental stone. When cut with a convex surface it displays a soft milky reflection, forming a luminous band, but not sharply defined as in cat’s-eye. The ordinary moonstone is a translucent variety of orthoclase known as adularia (see Orthoclase), whence the peculiar sheen has been called “adularescence.” The effect is probably caused by interference from twin lamellae, or by numerous enclosures of microscopic laminae, definitely orientated, and it has been suggested that these may often be flakes of kaolin due to incipient decomposition of the felspar. Practically all the moonstone of commerce comes from Ceylon, principally from the Dumbara district of the Central Province. It occurs as pebbles and irregular masses in the gem-gravels and clay-deposits, and is also obtained by quarrying an adularia leptynite, as described by Dr. A. K. Coomáraswámy. Very similar in some respects to moonstone is the chatoyant soda-felspar which was called by T. Sterry Hunt peristerite, from Gr. περιστερά, a dove, in allusion to the resemblance of its lustre to that of the bird’s neck. The original peristerite was from Bathurst, near Perth, Lanark county, Ontario, but it occurs also at Macomb, St Lawrence county, New York.