The metal silver is an element. However, PURE silver is almost never used for jewelry- too soft. Instead a mixture of metals is used to create what is called Sterling silver. Sterling silver is an alloy of silver containing 92.5% by mass of silver and 7.5% by mass of other metals, usually copper.
It is a mixture, because pure gold is too soft to make jewelery out of. So one must always mix it with another element, such as silver or copper. 24 Karat gold is pure gold, and anything under it is a mixture.