The purity of gold is measured in karats. If gold is 100% pure, then it is 24 karat gold (24K). In its pure form, gold is soft and malleable, but scratches easily and is unsuited for jewelry wear. Consequently, it must be alloyed with other metals to give it strength.
18K Gold - 18 parts gold, 6 parts other metals by weight (75% pure)
14K Gold - 14 parts gold, 10 parts other metals by weight (58.3% pure)
10K Gold - 10 parts gold, 14 parts other metals by weight (41.7% pure)
Made from yellow gold and various alloys and is not a true white metal. It is often plated with Rhodium, a white and reflective metal that provides an excellent protective coating. Rhodium plating will gradually wear off and require re-plating.
18K Gold is 75% pure, containing 18 parts gold and 6 parts other alloying metals. 14K Gold is 58.3% pure, containing 14 parts gold and 10 parts other alloying metals. 10K Gold is 41.7% pure, containing 10 parts gold and 14 parts other alloying metals.