When you buy gold jewelry, it isn't pure gold. Your gold is really an alloy, or mixture of metals. The purity or fineness of gold in the jewelry is indicated by its karat number. 24 karat (24K or 24 kt) gold is as pure as gold for jewelry gets. 24K gold is also called fine gold and it is greater than 99.7% pure gold. Proof gold is even finer, with over 99.95% purity, but it is only used for standardization purposes and is not available for jewelry.
So, what are the metals that are alloyed with gold? Gold will form alloys with most metals, but for jewelry, the most common alloying metals are silver, copper, and zinc. However, other metals may be added, especially to make colored gold. Here's a table of the compositions of some common gold alloys:
Composition of Gold Alloys 
