tarnish
n.a thin layer of dark stuff that covers a metal surface and makes it look dull. It is usually caused by air or water.
n.a thin layer of oxide or other substance that forms on a metal surface, causing it to lose its luster. Often used in the context of silver or gold.
“The silver spoon has a thin layer of tarnish.”
“To keep your jewelry looking bright, you should clean it regularly to prevent tarnish from building up.”
“The antique silverware was carefully polished to remove centuries of tarnish, revealing the intricate patterns that had been hidden beneath the darkened surface.”
From Middle English ternysshen, from Old French terniss-, stem of ternir (“to make dim, make wan”), borrowed from Old High German ternen, tarnen, from Proto-West Germanic darnijan (“to conceal”). Doublet of dern and darn.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.