Precious metals used in jewellery making are almost always mixed with other metals, to make an alloy. This gives it the necessary strength and flexibility, to make durable jewellery.
Sterling silver is an alloy of 92.5% by weight pure silver, and 7.5% by weight of other metals, usually copper.
Yellow gold is gold alloyed with silver and copper and sometimes zinc, and white gold is gold alloyed with silver and palladium. 9 carat gold contains 37.5% by weight pure gold, and 18 carat gold contains 75% by weight pure gold.