According to the United States Mint, 1964 was the last year that quarters were mass-produced with a significant silver content, before rising silver prices forced them to switch to a copper-nickel alloy. The 1964 quarter was not pure silver, but had a high silver content of 90 percent.
The 1964 Washington head quarter contained about .1808 ounces of silver. All quarters made in the United States had a high silver content beginning in the 1790s and until the Mint Act of 1965, which mandated the use of other metals. Quarters from this time period can have a significantly higher value than modern quarters.