Tarnish

1. (a.) To soil, or change the appearance of, especially by an alternation induced by the air, or by dust, or the like; to diminish, dull, or destroy the luster of; to sully; as, to tarnish a metal; to tarnish gilding; to tarnish the purity of color.
2. (v. i.) To lose luster; to become dull; as, gilding will tarnish in a foul air.
3. (n.) The quality or state of being tarnished; stain; soil; blemish.
4. (n.) A thin film on the surface of a metal, usually due to a slight alteration of the original color; as, the steel tarnish in columbite.