Historically, the metallic content of coins was extremely important. For a piece of coinage to actually carry any value, the metal that it was made out of needed to be worth something. However, as the price of precious metals used to mint coins (like silver) skyrocketed and worldwide economies evolved, some countries, like the U.S., stopped using silver in their circulation coinage.
The primary difference between silver and clad coins is their metallic composition. Most U.S. silver coins (dimes, quarters, half-dollars and dollar coins) of the past have a composition of 90 percent silver and 10 percent copper. Half-dollar coins between 1965 and 1970 were made of 40 percent silver and 60 percent copper. United States clad coinage generally has a copper core with an outer layer composed of copper and nickel.
Silver is significantly more valuable than copper or nickel. As of March 15, 2011, silver is worth $34.11 per pound. Compare that to copper at $4.07 per pound and nickel at $11.37, and it is easy to see why many countries have switched over to clad coinage. Obviously, collectors and investors like silver coinage over clad coinage, but clad coins do accrue numismatic value over time -- as is evidenced by the value of older five-cent pieces (which have been composed of copper and nickel since 1866).
The simplest way to tell if a U.S. coin is made of silver is to look at the date. Any dime or quarter coin minted after 1964 has no silver in it. Half-dollars contain a 40 percent silver between 1965 and 1970, but from 1971 onward, they too contained no silver. Eisenhower dollar coins with an "S" mint mark from 1971 through 1976 were also minted with some silver. Starting in 1986, the U.S. mint has produced silver coins every year specifically for coin collectors and investors. These coins have common face values (usually a dollar), but they cost and are worth much more than their face value suggests.
Silver coins usually weigh more than their clad counterparts. For example, a silver Washington quarter weighs 6.3 grams while a clad Washington quarter weighs 5.7 grams. Silver coins also visually age differently. The faces of old silver coins often tone slightly, and the ridges are a uniform silver as opposed the slight copper tint on the ridges of clad coins. The sound a silver coin makes when you drop it is also different than that of a clad coin. A silver coin has a distinctive ring when you drop it.