Never seen any Nickel like this before.. damage or something else? Its en coated in a brown-black coating which is deep into the alloy.
depending on what they have interactive with they can turn dark or other colors. I have a nephew who as a child had a nickel go through his digestive system . Talk about a dark nickel from the stomach acids.
Back in the 50's & 60's, we used to find them like this on the beach in the winter months in Ocean City, MD. Chris