Solid gold jewelry is actually only 100 percent gold when 24 karat. Other gold pieces are considered solid gold, but are alloyed with other metals, such as nickel, copper, zinc and others, to strengthen the gold and make it more durable.
Gold filled or gold overlay jewelry is composed of a solid layer of gold which is bonded with heat and pressure to a base metal. The gold typically must account for at least five percent of the piece’s weight, though this varies with the fineness of the gold (10 karat gold must account for at least 1/10th of the total weight, while 12 karat or higher must account for at least 1/20th of the total weight).
Gold vermeil is also known as silver gilt. Unlike other plated pieces, vermeil must have a sterling silver base and the gold coating must be at least 10 karat gold and at least 2.5 microns thick. While still plated jewelry, gold vermeil does have more intrinsic value, as sterling silver is a valuable metal in its own right.
Gold plated jewelry is made by depositing a layer of gold onto a conductive surface. The base metal piece is immersed in an electrolyte solution with particles of gold; a current is used to deposit particles onto the piece. Different varieties of gold plating, then, are defined by the thickness of the gold applied.
Heavy gold plate must be 14 karat gold or higher and be at least 2.5 microns thick.
Gold plate must be 14 karat gold or higher and be at least 0.5 microns thick.
Gold electroplate must be 14 karat gold and be a least 0.175 microns thick.
Note: A micron is a millionth of a meter or about 0.0004 inches. To help you visualize this, a human red blood cell is about 5 microns across and a human hair averages about 75 microns.