Yes, it is because there is an alloy in them. When they stopped minting silver coins they made a big fuss about how they would NEVER be rare, so do not worry your silly head about collecting them... I suspect the use of "junk" was cooked up in some dark room by PR people to try to prevent all the coins from disappearing overnight. If you have never read the speech Lyndon Johnson gave when he pulled the coins, it is pretty interesting. http://www.presidency.ucsb.edu/ws/?pid=27108