Seemingly normal black rock

Picture probably isn't much help but here it is: https://imageshack.us/i/pb5e2Ijfj

So looking at it and in the picture it looks like a perfectly normal rock. I thought maybe onyx. But while you can't see it in the photo, it has several scratches on it from non copper items (cement and a quarter) but the line under it is bright copper like a new minted penny. Am I being stupid or is this some strange rock. Oh and it went through a rock tumbler if that affects it. Found somewhere in Michigan not sure what part as I have had it for years.