PCGS 2006-P Colorado quarter with the reverse clad layer missing. The coin is copper on its reverse, because the clad layer was missing before the coin was struck. Missing clad layers are most desirable on the "state side" of state quarters, since that is the most interesting side of the coin.
PCGS 2000-P Massachusetts quarter missing clad layer reverse. The coin is copper on the reverse side due to the clad layer missing before strike. MS64 Red/Brown
2004-D Iowa quarter reverse missing clad layer. The quarter is copper on the reverse side, because the nickel outer later was missing before strike. A somewhat scarcer state for missing clad layers...
NGC 2006-P 25c Colorado quarter reverse missing clad layer. The coin is copper on the reverse side, and is missing the clad layer. A scarce state for missing clad layers. Desirable with the "state"...