At MGS, we tell people that we accept four kinds of metal: gold, silver, platinum, and palladium. But technically, since these metals can be alloyed in countless way, we actually accept lots of metals – so long as they contain any of the above listed metals.
The number of purity levels for precious metal alloys can be difficult to remember, so we’ve created this handy infographic that compiles the most common purity percentages. But before we dive into that some explanation of how purity is measured is helpful.
In the U.S. the purity of gold in an alloy is expressed in kt or karats. This is not to be confused with carats – a measure of mass for gemstones (to add to the confusion, “karat” is spelled “carat” everywhere else but the U.S.). Karat purity is measured as 24 times the pure mass divided by the total mass:
K = 24 (Ap/Am)
Ap equals the mass of precious metal in the Alloy, and Am equals the total mass of the entire piece of Alloy.
But there is more than one way to look at gold purity. What karats actually measure is called millesimal fineness. Millesimal fineness is the measure of a precious metal compared to other impurities in parts-per-thousand. For example, 24kt gold has a millesimal fineness of 999 or higher, while 18kt gold is only 750. Millesimal fineness stamps on gold products are more commonly used in other countries instead of the karat stamp – which is favored in the U.S. and Canada.