Yes.. Nickel is composed of Cupro-Nickel. 75% Copper and 25% Nickel. Exposure to the elements will tone the Copper. It is called Environmental Toning Damage. It is not 100% Copper.
Welcome to CT @Notyard and happy hunting. @paddyman98 has posted literally dozens of such coins he has found while metal detecting. While it is just an environmentally damaged coin, I think they are cool to put in a flip and keep as an example. Keep checking those coins. Here is mine.