Sterling is an alloy of silver containing at least 92.5% pure silver (925 parts of 1000 thus the 925 stamp), the remainder is made up of a mix of metals (most commonly copper) used to make the metal workable. Pure silver is generally considered too soft to work with, although you can find silver jewellery at 950 or 980, even the occasional 99.9% fine silver, which can be work-hardened to increase durability.