Tarnish was the result of an oxidation chemical reaction that caused to certain metals to corrode.
Worf commented to B'Etor in 2370, that "You cannot tarnish a rusted blade," after she claimed that the would be assassins of Worf implicated them in order to "tarnish our good name." (TNG: "Firstborn")
According to Grand Nagus Zek's 2371 revised Rules of Acquisition #24: "Latinum tarnishes, but family is forever." (DS9: "Prophet Motive")
Rule of Acquisition #263 stated: "Never allow doubt to tarnish your lust for latinum." (DS9: "Bar Association")