Ron Guth: World War II required vast amounts of strategic metals for the war effort. Among these was copper, which was being used in large quantities to create Cents for circulation. A number of substitutes were tested as replacements before the Mint settled on a zinc-coated, steel blank. When new, Steel Cents are exceptionally bright and flashy, but the zinc oxidizes quickly if the coin is not protected, resulting in a dull, powdery finish that comes off quickly in circulation, leaving a dark gray planchet underneath. Steel Cents were minted in the hundreds of millions, so they are still easy to locate today, even in high grade. Keep Gems away from humidity and they should retain their bright luster for decades.
By error, some bronze planchets made it into the hoppers at all three of the Mints, were struck and released into circulation. These have become the most famous (and valuable) of all off-metal errors.