For as long as coins have been used as money, they have almost always been made of metal. The only variable has been which metal (or combination of metals) was used. The United States has used a wide variety of alloys over the past 200+ years, some of which were tried on a temporary/experimental basis while others were used for decades.
In deciding on a coin’s metallic composition, there are two main considerations: value and durability. Small-denomination coins (like half cents, cents and two-cent pieces) have traditionally been made of copper or bronze. While dramatically less valuable than gold or silver, these reddish-brown metals are still regarded as having some amount of intrinsic value. Furthermore, these lower denominations were “workhorses” of everyday commerce—and copper/bronze were durable substances that could withstand heavy use.
When the US Mint was first established in the 1790s, all coins denominated from 5c to $1 were all struck in an alloy of 90% silver and 10% copper. The copper was added to give the coins additional strength; pure silver is quite soft and susceptible to wear. Furthermore, small/thin silver coins like half dimes would have been at risk of bending without the added sturdiness. Even with the addition of copper, silver coins are among the least durable of all coins made by the United States.
Like their silver brethren, America’s gold coins contain a small amount of copper for added strength and durability. “Vintage” U.S. gold coins (i.e. those made from 1795-1933) are 90% gold and 10% copper, but modern American gold eagles are actually 91.67% gold, 3% silver and 5.33% copper. Not only does this mix make the coins stronger, but it makes them a tad bit bigger and easier to handle.
Nickel coinage first appeared in 1865, as gold and silver were extremely scarce during the Civil War. Only three cent and five cent coins were made in nickel, but eventually the greyish metal found its way into many other denominations. Today, the dime, quarter and half dollar are made of a copper-nickel clad; they have the silvery appearance of a silver coin but are actually mostly copper with some nickel coating.
Perhaps the most famous and shortest-lived US coin alloy was the “steel” cent of 1943. In actuality these coins are made of zinc, but the public incorrectly assumed they were manufactured of steel. Copper and nickel were sorely needed for the war effort in 1943, so the Mint switched the penny from copper to steel. While it helped our country conserve copper, the zinc cents did not perform as well as originally hoped. The coins were extremely prone to corrosion and are often seen with ugly, blemished surfaces.