Most likely Environmental Damage - Buried Nickel exposed to Dirt, soil or sand Here are some I found Metal Detecting -
Ricardo, Paddy has been digging up coins for a couple of decades so his word is gold when it comes to “unearthed” coins. Unfortunately for paddy unearthing gold coins can be elusive! Reed & Sparkles.
I have a few Jefferson 5 cent coins with a coppery looking color and was told by a reputable dealer they were struck on improperly annealed planchets. Not saying that your coin is the same.