U.S. dimes, quarters, half-dollars, and dollar coins were made of 90% silver, 10% copper until 1965. Pure silver is much too soft to use for circulation coins (almost all silver jewellery and tableware is alloyed too), and is tarnished by sulpher. Many people were quite upset when the U.S. Mint switched to base metals, and some of them withdrew hundreds of dollars of quarters to hoard -- I inherited a brick-sized box-full from my grandfather, and I'm sure there are rolls of them in attics all over the U.S. This should keep old coins around, if not in circulation, for a while longer.
In 1980, when inflation and the Hunt Brothers caused a huge demand for silver, the metal value in old coins was higher than the face value, so many people sold their quarters, at high profit, to be melted down. Silver was about $4.40/oz in February 2002, but supply seems to be getting lower and demand higher, so there might be another crunch. Enjoy 'em while you got 'em.