A clad quarter weighs 5.67g - https://www.usmint.gov/about_the_mint/index583f.html?action=coin_specifications Even if the copper layer is missing, the difference is tiny. A silver proof weighs 6.25g (they are made to the 1964 and earlier specifications) http://www.coinflation.com/coins/1932-1964-Silver-Washington-Quarter-Value.html
I'm well aware of silver quarter years, proof mints etc... don't exactly have a scale to weigh at the moment.