It likely isn't silver, but it's likely plated in Platinum. The silver quarters are proof finish and done at San Francisco, so they have an S mint mark.
You didn't post a picture to be sure, but I can tell you that the
State Quarters and State park quarters were both plated and sold in a marketing ploy to the unsuspecting and there were gold plated sets as well as platinum plated sets.
You can look up gold or platinum plated quarters and see all of them being offered by various companies with P or D mintmarks. all of them are plated, and somione overpaid for them, took them to a coin shop likely, and were offered 25 cents for them so spent them.