Not necessarily, but first lets be clear '925' alone (without a full set of markings) doesn't mean anything. It's not a legally recognised mark. In fact you can buy a 925 stamp online for just a few pounds. The only way to be sure of a metals purity is to get it assayed by one of the recognised independent assay offices. Hallmarking is a huge topic that I will leave for its own post but basically owning a piece of jewellery stamped 925 neither proves or disproves it's silver.