Millesimal Markings

Q: I purchased a ring that has the number 417 engraved in the inside. The ring is supposedly 10k, but it doesn’t say so—what does 417 mean?

A: 417 is the decimal equivalent of 10K. What 10K means is 10 parts gold to 24 parts metal, or 10/24ths gold in the overall alloy (the other 14 parts will be a mix of copper, silver, and some zinc, for a yellow gold, or copper, nickel, and zinc for a white gold. If you convert the fraction “10/24ths” to a decimal percentage, you’ll find it’s 41.7 percent gold. In much of the rest of the world where the metric system is standard, gold purities are often expressed as parts per thousand; either instead of, or in addition to, the Karat content. Thus 417 means 10K, 583 means 14K, 750 means 18K, and so on. Note that this “millesimal” mark, unlike a karat mark, actually only specifies the percent of precious metal, without actually specifying the metal itself. So the same system can be used for silver, where 925 means sterling silver, or for platinum, where a 900 mark means a 90% platinum (the other 10% is usually iridium). Also, sometimes you’ll find pieces marked slightly above their actual karat percentage. For example, 14K is actually 583, but it’s often marked 585. In these cases, the metal is alloyed slightly above 14K, to the 585 percentage, in order to allow for solders, or errors, etc, and still pass the often stringent karat standards in use in some countries. (The U.S. standard allows a trace below actual karat content for those errors or solder variations, etc., but not all countries do.)

by Peter W. Rowe M.F.A., G.G.