Not all gold jewelry or trinkets are created equal. They are all made with different levels of purity, which determine their overall worth.
In the U.S. the purity of gold in a piece is expressed in kt or karats (not to be confused with carats – a measure of mass for pearls and gemstones). The highest karat is 24, hence 24kt gold jewelry is pure gold. Gold with a lower karat count, such as 18kt, contains impurities. These impurities are added to increase durability, alter color, or cheapen the gold.
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 gold (or other 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 over the karat stamp, which is favored in the U.S. and Canada.
The use of millesimal fineness has created some lingo among jewelers and refiners. You may hear a piece of jewelry is “four nines fine” or “one nine fine.” These would be 999.9 and 900 in millesimal fineness, respectively. Chances are though, you’ve never come across the elusive “six nines fine.” At 999.999 millesimal fineness, it’s the purest form of gold ever produced.