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the variegated, often iridescent, coloring of a thin surface layer of a mineral, differing sharply from the color of the rest of the mineral. Tarnish is caused by the presence, on the surface of the mineral grains, of a thin film formed as a result of the alteration of the mineral, for example, under exposure to oxygen. The film often produces an iridescent effect. Tarnish is typical of bornite, chalcopyrite, limonite, and other minerals. It is not observed on a fresh surface of mineral breakage.