The U.S. Mint never made "pure" silver coins because 99.9 percent silver is too soft to use in circulation. Dimes, quarters and half dollars were minted with 90 percent silver content through 1964.
Dimes dated after 1964 are copper-nickel clad and contain no silver. Quarters are also clad, except for the bicentennial commemorative coins with an "S" mint mark that were produced for mint and proof sets. Kennedy half dollars from 1965 to 1970 were 40 percent silver composition. After 1970, all Kennedy halves were clad, except the same "S" bicentennial issue. The U.S. Mint restarted dollar production in 1971. Circulating Eisenhower dollars were clad. The 1971 through 1974 "S" coins were 40 percent silver, as well as a bicentennial "S" version.