18 karat gold is one of the most popular alloys for jewelry. But how can you recognize that a piece is indeed made of gold with this level of purity? Here we have listed the most common marks for 18K gold that you could find stamped on such jewelry.
The Meaning of 18K Gold Purity
18K gold is an alloy of relatively high purity. As its karat number suggests, it consists of 18 parts pure gold and 6 parts other metals, for a total of 24 parts.
In terms of percentage, 18-karat gold contains 75% pure gold.
Karat Marks for 18K Gold
The standard karat marks for 18-karat gold are easy to recognize – they contain the number 18 followed by letters such as K, Kt, or KT, all of which indicate karat.
So, if you see a stamp that reads 18K, 18KT, or 18Kt, it means the gold is 18 karats.
A more interesting case is when you see the letters KP (also K.P.) after the karat number, as in 18KP.
The letter P stands for “plumb”, and the mark is read “eighteen karats plumb.”
This means that the gold content in the item is guaranteed to be no less than 18 karats.
The P-letter mark is important because unless it is stamped on your jewelry, its actual karat may vary slightly from the mark value – the permitted deviation is 0.5 karats, so an 18K piece might actually be 17.5 karats.
Parts-per-Thousand Marks for 18K Gold
“Parts per thousand” is a unit that is sometimes used instead of karats, especially in jewelry made in Europe. The difference is that instead of calculating purity per 24 parts of gold alloy, as in karat notation, it is calculated per 1000 parts.
The common parts-per-thousand mark for 18K jewelry is 750. You might notice that this number is equal to the percentage equivalent of 18 karats (75%) multiplied by 10.
In general, to convert a parts-per-thousand number to karats, divide by 1000 and multiply by 24.
Marks for 18K Plated Gold
If you see the number 18 stamped on your jewelry, this does not necessarily mean that it is made of a solid gold alloy. Sometimes, a piece is actually made of another metal and is only covered with a thin layer of gold.
Although it looks like gold, such jewelry is gold plated.
The most common mark for gold-plated jewelry is GP or G.P. If you see a stamp such as “18K GP”, this is an indication that the item is only plated with 18-karat gold, not made entirely of it.
Other marks that denote gold plating are:
- RGP – rolled gold plate
- HGE – heavy gold electroplate
- HGP – heavy gold plate
- GEP – gold electroplated
So, if you see a stamp such as 18K RGP, 18Kt HGE, 18KT GEP, or 18k HGP, then you have jewelry that has 18-karat plating.