Silver stamped ".925 FAS" is made of 92.5 percent silver, and the remaining 7.5 percent can include a combination of other alloy metals. These other metals may include copper, nickel or zinc.
Silver is not required to be marked in the U.S. This type of FAS silver is also called sterling silver.
A drop of nitric acid can test a piece of jewelry to determine whether it is sterling silver. The nitric acid turns a creamy color if it is authentic sterling silver, and it turns green if the jewelry is nickel silver, silver-plated brass or a low-quality silver alloy.