Surface looks like rust

Can anyone tell me what may have caused this nickel's surface to appear brown? (paint, environmental, chemical, oxidation...?)

 

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Probably been in the ground. There's another thread about this: https://www.cointalk.com/threads/buffalo-nickel-rust-or-not-nickel-made.373188/

 

That coins been in the ground for many years. A true metal detecting find.

 

Definitely long-term environmental exposure (Dirt) toning damage. That Buffalo Nickle is composed of cupronickel. I have found many just like yours when metal detecting.