Tarnish on an old portuguese silver coin?

Hello group! I recently noticed this brownish/reddish spot on my 400 reis (1798) silver coin. Is that tarnish or copper oxidation? Thank you in advance for your answers.

Isn't oxidation a form of tarnish? Are you asking if silver or copper specifically had undergone a chemical reaction? 

In any case, probably need a microscope or a better macro lens to see that tiny spot clearly.