Ryan, It being a "Clad" Quarter, the outer coating is nickel, which will discolor when it's been in the ground, Quarters and dimes now being made of nickel over copper, 1964 was the last year American change were minted in Silver, "with some exceptions", and if your quarter had been in the ground long enough, it may very well have been brown, or even reddish in color, depending on mineral content, how wet the ground, any fertilizer etc. and so no, it's not rare, but I tell ya, keep hunting, sooner or later you'll find one of 1964 or earlier, and you will have Silver!