The quality of silver is measured in part per one thousand. Sterling silver is 925 part per 1000 or better. Fine metals like silver, gold and platinum are stamped in ways that indicate the purity of the metal. Sterling silver is marked "925" or "Sterling". There is Britannic silver which is 954 part per 1000 or better. Typically, silver is mixed with copper to make it a little harder, and copper is what tarnishes.
In the case of gold, pure gold is 24 karats (KT). more often you find gold at 18KT (75% gold) or 14KT (58.3% gold. Yellow gold has equal parts silver and copper mixed with the gold. Green gold is mixed with only silver and pink gold is mixed with only copper. White gold is an alloy made in two ways: the most common is with palladium and there is another alloy made with copper, nickle, and zinc.