Jewelry made of silver is always marked with a small stamp that indicates its authenticity. Sometimes, the maker stamps its name or logo on the piece. FAS is another mark often found on silver jewelry.
The FAS mark appears on sterling silver jewelry that is also labeled as 925. FAS stands for fused alloy silver, meaning that a piece of sterling silver jewelry is infused with an alloy. Sterling silver therefore may have a stamp of FAS 925.
In silver jewelry marked with the FAS 925 stamp, the number indicates that the jewelry is 92.5 percent silver and the rest is a fused silver alloy. Sterling silver, or 925 silver, contains 92.5 percent weight of silver, as compared to 99.9 in pure silver. The rest of the weight in 925 silver is typically copper, which strengthens the silver.
FAS may also stand for a company that created silver jewelry, Fantastic Antique Silver. This company no longer exists, but several pieces of its jewelry are still sold in certain places. If FAS is stamped on jewelry which isn't antique, it most likely is a reference to fused alloy silver.