iridescent gemstone in the plagioclase (q.v.) series of feldspar minerals. The name (from Greek peristera, “pigeon”) refers to the resemblance of fine specimens such as those from Ontario and Quebec to the commonly iridescent feathers of a pigeon's neck. In peristerite—usually a form of one of the sodium-rich varieties of plagioclase albite or oligoclase—the sodium aluminosilicate and calcium aluminosilicate that make up the mineral separate and form intergrowths of submicroscopic but distinct crystals of the two compounds. The iridescence probably arises from diffusion of light by adjoining areas of different optical properties, or from reflection and diffraction of the separating crystals of the two different substances.