Whoa, someone went to town on this one. I agree with what's been said- the edge is due to acid exposure, which preferentially corrodes the pure copper core over the copper-nickel cladding. It also appears to what we call a "campfire coin," with a large bubble produced by heating.
In the future, try to avoid holding the surfaces of coins you suspect to be valuable. It's obviously not a problem here, but on high-grade coins, the oils in your skin will eventually create fingerprints that damage the value.