Hello Drwhy, and welcome to the forum!

I agree, environmental damage, possibly a metal-detecting find.
Some error quarters do exist in which one or possibly both of the copper-nickel clad layers were missing from the original planchet. The resulting struck coins will show a brownish copper surface instead of the normal silvery color.
A coin missing a clad layer will be lighter than normal by at least 1 gram. If your coin is close to the standard weight for a clad quarter (5.67 grams) you can be very sure it's nothing special.