18-karat gold jewelry is made of a gold alloy that contains 75% gold and 25% other metals, which make your jewelry harder than the overly soft pure gold.
You can find out the purity of your gold piece by either looking at its karat stamp or testing it for its actual gold content.
Karat Stamps on 18-Karat Gold Jewelry
18-karat gold ring
The most straightforward way to check the karat of your gold jewelry is to look at its surface and see if there are any markings. Gold jewelry usually has its karat number stamped on it.
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.
Sometimes, however, a piece of jewelry sold as 18 karats might have no markings at all.
Or, even though an item is marked as an 18-karat piece, it might actually have lower purity. In such cases, the best way to check the gold content of your jewelry is to subject it to acid testing
Testing 18-Karat Gold with Acid
Acid testing is a way to determine the purity of a gold alloy by putting it in nitric acid and observing the reaction. By the change in the color of the alloy one can establish its gold content.
You can have acid testing done on your gold at a jewelry store for a fee. Usually, the jeweler will take a small sample from your piece to test it.
This is usually done by scratching the item in a place that is not prominent, and the procedure doesn’t leave readily visible traces on the jewelry.
You can also do this test yourself by purchasing a gold testing kit.
Here’s how you can use such a testing kit to see if your gold is really 18 karats:
1. Scratch your jewelry – usually this is done by rubbing it on a testing stone to leave a gold mark on it. Don’t rub your jewelry in highly visible places.
2. Rub the testing needle labeled “18K” on the stone, next to the gold mark – the needle should leave a mark, whose color you will use to compare with that of the gold mark after it is treated with the acid.
(If your testing kit doesn’t come with karat needles, don’t worry, and proceed to the next step.)
3. Apply some of the acid that is labeled “18K” to the gold scratch mark and the needle mark – this acid is made with the exact concentration needed to react with 18-karat gold.
4. Observe the color of the gold mark. If the gold is 18 karats or higher, its color should not change significantly, if at all. It should look the same as the color of the 18K-needle scratch mark.
If your jewelry is less than 18 karats, its scratch mark should become faint and disappear gradually. If the jewelry is only 1 or 2 karats less than 18 karats, then the mark’s color will become somewhat rusty but not disappear.
Note: Depending on the testing kit you purchase, the testing methodology may differ from the steps described above, so make sure to read the instructions that come with the kit.
Where to Buy Real Gold?
For jewelry with diamonds, emeralds, rubies, or sapphires, we recommend James Allen (read review) because you can take a 360-degree look at any stone before having it set in gold.
