United States Clad Coinage
In 1965 all the coins that traditionally had been made from an alloy of 90% silver and 10% copper were replaced by base metal coins. The Mint adopted a clad "sandwich" of copper and nickel to replace the coinage of dimes and quarters and a clad sandwich of 40% silver and copper to replace the half dollar (the half dollar dropped the silver content in 1970 and adopted the base metal clad metal in 1971).
Below is a comparison of the old silver coins to the current clad, copper-nickel versions
All dimes, quarters and half dollars are currently minted on blanks cut from the prescribed clad strip that consists of outer layers of copper-nickel (75% copper and 25% nickel) bonded to an inner core of pure copper. This concoction isn't always stable and if gas or dirt comes between the layers, one or both of the outer copper-nickel layer may fall off when the planchets (blanks) are cut from the clad strip. The resulting blank may have a "yellow" or "red" side and a "silver" side. These incomplete error blanks are struck with the coin's design and become a mystery to those who find them in circulation. Clad coinage completely missing one or both layers are very collectable and bring a premium when offered in the numismatic community. These errors are called lamination errors and can also appear with only part of the outer layer missing. Small lamination errors actually reduce the coin's collector value.