The Office of Weights and Measures (OWM) has prepared a Conversion Factors for Precious Metal Sales chart [PDF] to assist consumers and businesses in facilitating value comparison. State weights and measures programs are encountering an increasing number of scrap gold buyers and sellers that use scales which weigh in milligrams (mg) and grams (g). The carat (c) is another common measurement unit used in jewelry sales to measure gemstones. The carat had its origin in the carob seed, which later standardized a 1/444 ounce and then 0.2 gram (or 200 mg).
220.127.116.11. Precious Metals. - Gold, silver, platinum, or any item composed partly or completely of these metals or their alloys and in which the market value of the metal in the item is principally the gold, silver, or platinum component.
2.17.2. Quantity. - The unit of measure and the method of sale of precious metals, if the price is based in part of wholly on a weight determination, shall be either troy weight or SI units. When the measurement or method of sale is expressed in SI units of mass, a conversion chart to troy units shall be prominently displayed so as to facilitate price comparison. The conversion chart shall also display a table of troy weights indicating grains, pennyweights, and troy ounces.*NIST Handbook 130 Uniform Laws and Regulations in the Areas of Legal Metrology and Engine Fuel Quality. Uniform Regulation for the Method of Sale of Commodities, Section 2.17.