Your coin may have been heated super hot. Which makes the two metals separate and allows the copper to come out to the surface of the coin, which will change its color. Nickels are 75% copper and 25% nickel. So yes a burn of sorts.
I don't see a mechanism for bringing copper to the surface by heating it. I think that's just oxidation. You can preferentially dissolve nickel out of copper-nickel with acid, leaving a more copper-colored coin.
Better pic needed to make an honest assessment. In short, from your pic you can't really tell if it is copper or not.
I agree, Environmental damage. More than likely from a coin that was found metal detecting or been in the soil.
Actually I would say the nickel is almost black instead of a copper tone. It is much darker then a normal penny color.
This coin is in smooth conditon like a regular nickel it almos have a black color instead of the regular copper look.
Your coin probably looks that way because it suffered from some type of harsh exposure to the environment at sometime in the last sixteen years. Sun,heat,dirt, harsh chemical, etc.
Ray.. he's just kidding with you. The answer that it is "dug" is most likely correct. The Nickel composition has a bit of Copper in it. As stated, when Nickels are buried under certain types of dirt they can change color. Think of it as toning caused by exposure to the outside environment. I myself do Metal Detecting and have found hundreds of Nickels that look like yours. No need to keep them so detectorists spend them and they eventually end up in a register for someone like you to get it back as change.