San Francisco hasn't made a single clad coin up until recently. (I think 2012?) Besides then, the last time they made clad coinage was in the 60s. so this one is silver.
All proof sets from the 1970s and early 1980s (except the silver bicentennial sets) featured S-mint clad dimes, quarters and halves. All state quarters were minted in both clad and silver issues bearing the S mintmark. So are the ATB quarters. All that aside, the OP's quarter certainly looks like silver. It's possible that it's one of those dumb platinum-plated novelties, but I doubt it.
Actually, this is false. (the one about the state quaters also being minted in clad as well.) Not only have never seen nor heard of a clad state quarter from SF, i have a red book which states that the SF mint minted its first clad quarters since 6? in 2012? (but not with the states.) I am the proud owner of two of these clads, and neither of them are states. Pray tell, were did you get your info?
Heres a price guide, from pcgs, clad s mint mark is included http://www.pcgs.com/Prices/PriceGui...P=1&c=720&title=Washington+50+States+Quarters
I was wrong, i must have read it wrong or something. any who, this coin is silver, or plated. But has some silver content.
I don't have the latest Red Book, but I imagine they were talking about non-proof quarters. Yes, SF started minting these in 2012 -- "circulation strikes", but not released for actual circulation, just sold in rolls and bags to collectors.
If you want to be sure weigh the coin. Silver will be around 6.25 grams, a plated clad would be around 5.67 grams.
This is kinda late but I found an older scale to weigh this quarter. The scale measures in grains and it weighed 98.5 grains. I converted it to grams and its 6.38 grams. Conder101 thanks for the info on what it should weigh.