Modern US coins are made from base metals such as copper, nickel, and zinc. The face value of the coin (how much it is "worth") is usually greater than the intrinsic value of the metal. However, over time, inflation may cause the value of the metal to rise so that the face value becomes less. For example, US pennies minted before 1982 actually contain more than a penny's worth of copper!
Virtually all US coin denominations have been in use since the late 1700's. The penny, which is the oldest non-gold denomination, was first minted in 1793. The five cent nickel is a relative new comer, first appearing in 1866.
Since all denominations above the nickel were originally made from precious metal, they have reeded (grooved) edges. This makes them difficult to counterfeit and it is hard to file down the edges (thus stealing precious metal) without detection. Today, the reeding is little more than a cosmetic feature, although it does help the visually impaired to distinguish coins based on touch. For example, the penny and dime are of similar size, but the grooved edges of the dime make it easy to pick out without seeing it.
The United States government currently operates mints in Philadelphia, San Francisco, and Denver. The Philadelphia mint was the first to be opened. The original building was constructed in 1792 (when Philadelphia was the Capital of the United States) and coin production began the following year. The current Philadelphia mint is the fourth such facility in that city and was opened in 1969. The Philadelphia mint produces coins for circulation and, until 1968, it also produced nearly all US proof coins.
The Denver mint opened in 1906 and produces coins for circulation. Most of the currently dated coins circulating in the western United States come from this facility.
The San Francisco mint was originally established in 1854. It closed for a time in 1955, but was reopened during a coin shortage in the 1960's. In 1968, the San Francisco mint took over the production of most US proof coins and, since 1975, has been used almost exclusively for this purpose (although Susan B. Anthony Dollars and some Lincoln Pennies have been produced here for circulation).
Current US coins are the: Lincoln Penny, Jefferson Nickel, Roosevelt Dime, Washington Quarter, Kennedy Half Dollar, and Sacagawea Dollar.