A gray, blue, green, or brown plagioclase feldspar with composition ranging from Ab₅₀An₅₀ to Ab₃₀An₇₀, where Ab=NaAlSi₃O₈ and An=CaAl₂Si₂O₈; in the course of formation when the natural material cools, the feldspar sometimes exhibits a variously colored luster. Also known as Labrador spar.