When buying gold jewelry, always look for the karat mark. All other things being equal, the higher the karat, the more expensive the piece.   In the United States, 14-karat gold, or 583 parts pure gold, is the most common degree of fineness.   Nothing less than 10 karats can legally be marked or sold as gold jewelry in the U.S. However, lower karatages, such as 8-karat gold and 9-karat gold, are popular in other countries.   18-karat gold is 18/24ths, or three-quarters pure gold, and jewelry of this fineness is marked 18k or 750, the European designation meaning 75% gold.   Always look for the karat mark or "k" that appears on the back of the piece. By U.S. law, if a karat mark appears you should also see the manufacturer's trademark to assure you that the karat marking is accurate. The country of origin should also appear.   In addition to the karat mark, every piece of gold jewelry should be stamped with a hallmark or trademark of its maker, and sometimes its country of origin.   These designations assure you that you are buying genuine karat gold jewelry. Heavier pieces contain more gold.   Gold Filled, also called Gold Overlay, refers to a layer of at least 10-karat gold that has been permanently bonded by heat and pressure to one or more surfaces of the support metal, then rolled or drawn to a prescribed thickness. The karat gold must be at least 1/10 of the total weight.   Gold Plate means that a layer of plating of 10-karat gold or better has been bonded to a base metal. The karat gold content may be less than 1/20, but it must be properly identified by weight in terms of total metal content.   Gold Leaf is just gold plating that's been pounded and applied by hand.   Vermeil refers to gold plating that's at least 15 microns thick (one micron is a millionth of an inch).