I believe it is just environmental damage/toning/ rust. When you are missing a clad layer it is almost 100% on one side only. Missing both layers would be VERY thin, VERY light, and the coin details would not be regular as shown here. If this weighs 5.67g it's worth a quarter.
I find coins like these metal detecting. The soil/ elements/ minerals/ chemicals/ pollution along with moisture turns them this color. Here's one from last week. It's the same color on the obverse:
It's actually Cupro-Nickel. The Copper tones after harsh exposure. Here are some of my metal detected examples.. Nickels are also comprised of Cupro-Nickel.