Silver is a soft metal in its pure form, too soft to hold up against the rigors of jewelry making and even wearing so it's mixed with other metals to make it more durable. This silver mixture, called an alloy, is known as Sterling Silver. Most Sterling Silver on the market today is made up of approximately 925 parts per 1000 silver and the other 75 parts per 1000 copper, however, the copper has a tendency to tarnish which is the darkening that occurs when Sterling Silver reacts with gases in the air. This is why it is good to clean your silver regularly.