I work in retail so quite a few coins pass through my hands. A while back, I opened a roll of quarters and noticed one that looked different so I snagged it. It seemed like it was a different metal than usual and no copper layer was visible. It sounded different and felt different when I tapped it on a tooth. Interesting. Tonight I broke out my reloading scale and weighed it and discovered that it was a few grains heavier than other state quarters. This prompted me to look at it more closely and I noticed that it has an "S" mint mark. I just did a bit of poking around and found that the only quarters that came from San Francisco are the proof coins for sets. I imagine that this one was part of a set and either someone was hard up for cash or the set was given to a kid who opened it because he wanted to get a can of soda. Sadly, it's not in proof condition anymore. Are the proof coins made out of the same clad metal as the regular coins?