I usually Collect Cents but i came across this Dime after finding a Bag of Coins in a Trailer i was Cleaning out
The picture doen't really look copper, but I imagine it's different in hand. My guess is the planchet skipped its nickel plating.
Well the plating isn't very thick anyways so you couldn't tell by looking at it (not sure why I asked that come to think of it ) . But if it truly is copper that you are seeing then it's just unplated. Are both sides like that? Dimes are made from a copper-nickel plating over a solid copper core so that looks like the core that you have there.
Most likely just discolored. If it was truly missing the clad layers it would be significanly underweight, and the strike would also be weak due to the thinness of the planchet. This piece does not show the weakness that should be expected.