Tarnished Silver Tarnish is in no way indicative of a low quality in sterling silver. Sterling silver (or .925 silver) is, by law, at least 92.5% pure silver and no more than 7.5% other metals. Sterling silver is an alloy, or mixture, of metals. Pure silver is too soft to be used for most jewelry and household good purposes, so other metals are added to strengthen the metal and make it more durable. Pure silver oxydizes (tarnishes) very slowly, but the addition of other metals to pure silver cause it to oxydize (or tarnish) more quickly. Laws regulate how metals must be marked. In the US, only pure silver can be marked “.999” and only silver of at least 92.5% purity can be marked “.925” or “sterling.”