There are a lot of environmental issues that can cause color change of cupronickel alloys from typical silver-white to copper. Proceed knowing this could be the case.
I saw some dug quarters on ebay the other day...they look just like coins that have been in the ground for some years. I saw one that looks like it had been tumbled clean mostly as well. These guys are selling them for $75 each and crap like that. Normally I would say unbelievable, but our society is sucked into so many scams....