Throughout history, gold has been considered a precious metal and one of its primary uses has been to make jewelry and decorative objects. The karat measurement refers to the purity of gold. In jewelry, 10 karat is the lowest purity (and the densest), while 24 karat is very soft and nearly pure. Sometimes that karat purity is marked on jewelry, but a simple test can resolve any questions.
Learn what karat refers to. Because gold is a soft metal, it is alloyed with other metals to improve its durability and strength. When it is alloyed with other metals, the karat measurement is used to indicate the amount of gold present. Twenty-four karat is pure gold and it is alloyed down to 10 karats. Items less pure than this are sometimes called gold-filled, gold plated or gold washed.
Look for markings on the piece. Typically, gold jewelry is marked with a number plus the letter K, to signify the karat purity. So, 10K is 10 karat gold and 24K is 24 karat gold. This will typically be on the clasp for necklaces and bracelets, on the inside of a ring and on the post of earrings.
Use a gold testing kit. A gold testing kit uses acids to determine the gold purity. A typical testing kit has separate bottles for 10K, 14K, 18K, 22K and 24K gold. To use, scratch the jewelry on the stone that is provided in the kit. Scratch in a protected area, such as the inside of the clasp, as it is a destructive test. The kit will have instructions as to the exact usage. The length of time the acid takes to dissolve is an indicator of the gold's purity.