The best definition of tarnished is

Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air.

If your talking about a person, tarnished means to lose respect, if your talking about stones or something non living, it means to lose luster or shine.