Tarnish

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.

To lose luster; to become dull; as, gilding will tarnish in a foul air.

The quality or state of being tarnished; stain; soil; blemish.

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.

discoloration of metal surface caused by oxidation