Review of Gold Marks
Pure gold, also called fine gold, is 24 Karat (or 24K). Such a mark represents a ratio or proportion of gold to other metals which may be present with the gold. This term should not be confused with carat which is a unit of weight for gem stones.
Alloys of gold are always expressed on a basis of 24 parts. Pure gold, with no additives, is 24K; 14K is 10 parts other metal to 14 parts gold. Gold less than 10K cannot carry a karat mark or be legally called gold in the United States. In Europe, most jewelry is a minimum of 14K.
In addition to, or in place of, the karat mark European jewelry commonly carries a separate number, or fineness mark. A fineness mark expresses gold content as numeric proportion to base metal. The fineness of 14K gold, for example, is expressed as .5833. On authentic 14K marks from Italy, this is rounded off to 585. The fineness of 18K gold is .7500; 10K is .4167.
Items described as gold filled are made of a base metal to which a layer of gold has been attached. The gold in this layer is a minimum of 10K. The common practice is to use 10 percent (by weight) of 12K gold to make gold filled layers. This means the actual gold on gold filled items is 5% pure (12K= 50% gold content; 10% of 50%=5%). Gold filled products are commonly marked "1/20th GF", "1/20th 12K GF" or "1/20 12K Gold Filled". This is because 5% is 1/20th of 100%. Rolled gold is the term for a layer only half as thick, or 1/40th which means the gold is only 2½% pure.
Often the mark indicating gold filled will be removed and another mark for a higher karat will be substituted. Where forged marks are suspected, the fineness of the gold must be tested. Testing kits for gold are available at most jewelry supply wholesalers.