Sorry for the slow response but I was going through all of my top shelf coins to pick you a fitting prize. Now everyone please don’t burn up Chris’s inbox at once about this very valuable beauty! This is local pickup only
Oh it’s electrolysis judging by the worn and faint metallic crystals of nickel in the clad layer on the coin using sulfuric acid to turn into metallic, inorganic sulfate salts. Don’t worry I have a coin like this it’s a 2000P penny with dentritic copper crystals on the plating