Gold is one of the most highly valued precious metals, but not all gold is created equal. The karat scale, the traditional measure of gold purity, indicates the percentage of gold to alloy. Twenty-four-karat gold is pure; other common purities are 8, 14 and 18 karat. Although most gold is stamped with a small karat number (followed by "K") that can be read with a magnifying glass, you need to take the examination a step further in order to ensure you're getting the gold you paid for.
Use a magnifying glass or loupe to find the karat indicator on the gold piece. This mark is not required by law, but careful buyers will insist on dealing only in gold with a karat indication, which appears as a number followed by the letter K. Although karat is the traditional measurement, the millesimal fineness system is also commonly used. This indicates purity on a scale of 001 to 999. An indicator of 750, for example, means gold that is 75 percent pure (meaning that it's 18K gold -- it's 18 of a maximum 24 parts, or 75 percent, pure). Buyers and jewelers consider a number from 995 to 999 as an indicator of pure gold.
Use a gold testing kit to confirm the purity mark. These kits include small testing stones, bottles of testing acid and a set of needles marked with the common karat indicators. Follow the directions that come with the kit. One common method is to swipe the gold on a testing stone, and then add the appropriate acid solution to the mark. Adding 18K solution to a scratch made with an 18K piece, for example, will cause a color-change reaction. If the gold is of lower purity, there will be no reaction. Tests of lower purity can also be done by making scratches with the pieces and the appropriate needles, adding the solution, and waiting for the marks to disappear. If both scratches disappear at the same time, the purity matches.
Bring your rings, jewelry, or other gold pieces to a retail jeweler or gold dealer and request a test (also known as an assay). Electronic analyzers can determine the purity without scratching or degrading the piece, or forcing you to bother with test kits and acid solutions. The assayer may charge a fee, but a written record of the results will assure you of the best possible proof of authenticity and purity for the piece you hold.
- Due to international standardization, small gold bars (less than 1 kilogram) made since 1994 are of uniform 100 percent purity. Before that time, gold bars were made by various methods and some have a purity rating of very slightly less than 100 percent.
- Various methods and materials are used to create pieces that only resemble gold, but are fake. To test authenticity, scratch the piece across unglazed tile; a black mark indicates pyrite (the yellow "fool's gold"). Also, real gold, like all precious metals, does not react with magnets unless it is mixed with a magnetic alloy. Lastly, gold is softer than glass, so any material that scratches glass is not gold.
Founder/president of the innovative reference publisher The Archive LLC, Tom Streissguth has been a self-employed business owner, independent bookseller and freelance author in the school/library market. Holding a bachelor's degree from Yale, Streissguth has published more than 100 works of history, biography, current affairs and geography for young readers.