think I found something! 1904 indian head

3/4 inch or 12cm I dont have a mm ruler. it is the exact size of a normal penny. if it were a chemical reaction, then why would it be lighter than a normal one? some size but a tiny tiny bit thicker than a normal penny.

DVCollector, that's VERY interesting...do you have photos of any examples?

unless it sat next to aluminum for any period of time alum and copper make aluminum oxide which would transfer aluminum onto the copper but it would take like 50 years

Very, very interesting

to the forum. Since you are new here I must say I am no expert collector nor am I even a novice in error coins, so I am learning mainly from posts that have been posted here. The surfaces look kind of grainy like it has been chemical treated like some of the others that have been treated with acid and other things shown on this site before. I am not saying that is what this is I am just saying the surfaces kind of remind me of those coins that have been posted that had been treated with the acid and other chemicals.