Man, that's a tough one. Guess I would be tempted to soak that one either in hydrogen peroxide and/or mineral oil. It looks to be copper. Might be a token. Hope you can get some detail of some sort on it.
It might be a 1/2 real as they are smaller than a dime and often quite worn. Sometimes come out of the ground black. Does it sound like silver or copper when dropped on a hard surface? If silver, wet it and wrap in in aluminum foil a few times to clean it.