Skip To Content

Gold Purity: A Complete Guide

Catherine Tramell
Catherine Tramell

Last updated 23. october 2021

Buying gold is easy — if you have the money, that is. Ascertaining the quality of the gold or its purity can be challenging, however. In simpler terms, determining the amount of gold in a given piece of gold is the real task.

“Ascertaining gold purity” is not finding out whether the article on hand is the real deal. The objective is instead to find out how much gold is there in a given piece.

A gold item – for instance, gold jewelry or gold bars – would go down as “gold” even if it’s not wholly pure or not entirely gold. The gold item must have a minimum quantity of gold (8K or 10K) to be sold as gold.

Quite clearly, gold purity is not a straightforward topic. Multiple variables need to be considered, such as the karat system, dealers, form of gold, etc. In this article, you’ll wade through the following to get a firm grasp on them all:

  • Karat system and millesimal fineness (both measures of gold purity)
  • Ways to measure the purity of gold (acid testing, electronic testing, etc.)
  • Answers to some critical questions and more

Gold is a highly valued precious asset, and the demand for it is always strong, both in the industry and among individual consumers. It, therefore, pays to take a deep dive into the topic.

The Karat System

If the discussion is “gold purity”, it must start with a basic understanding of the “karat”.

The term “karat” could mean multiple things. In the gold context, it denotes “purity level”. The higher the karat number, the purer the gold or a lesser presence of other metals.

For a gold article to qualify as “gold”, it should be at least 10 karats. That means 41.7% of the item’s makeup should be gold and the remainder other metals. The karat gold numbers go up from there, up to 24 karat gold (100 percent gold). In some countries, the minimum level needed could be lower at 8K. 10K is more universally accepted.

The following is a chart listing out the karat numbers and what do they denote (in terms of elemental composition):

KaratsGold (%)Silver (%)Copper (%)Palladium (%)Zinc (%)
937.542.520--
1041.7526.3--
1458.38.2033.5--
187512.512.5--
2291.752-1.3
24=>99.9----
937.562.5---
1041.747.4-100.9
1458.332.2-9.5-
1875--25-
937.52042.5--
1041.72038.3--
1458.39.232.5--
18752.7522.25--
2291.67-8.4--

Popular color variations of gold, such as rose gold and white gold, would not have been possible if the purists insisted on pure gold jewelry or not mixing the precious metal with other metals. Not to mention, gold is alloyed with other metals for increased strength and rigidity.

The Fineness of Gold

In the precious metal context, “fineness” is the amount or weight of the primary metal in relation to the piece’s overall heft. The fineness is typically expressed as karats, as explained above, and also as “millesimal fineness”. “Purity” and “fineness” are essentially the same thing.

A karat is one part per 24 parts. Millesimal fineness measures or expresses a precious metal’s fineness in 1000 parts. For example, a piece of 18 karat gold contains 75 percent gold. Its millesimal fineness would be 750. The millesimal system is commonly used in Europe. The U.S. and the United Kingdom use karats more.

The millesimal fineness system can help denote the purity of other precious metals — such as platinum and silver. The following are some of the gold purity numbers under the system and what they represent:

  • 999.999 (six nines fine): Purest gold or type of gold to be ever produced (Refined in 1957 by the Perth Mint)
  • 999.99 (five nines fine): Purest form of gold currently made (Commemorative coins produced by the Royal Canadian Mint)
  • 999.9 (four nines fine): The most prevalent gold (American Buffalo and Canadian Gold Maple Leaf coins)
  • 999 (three nines fine): 24 karats (Chinese Gold Panda coins)
  • 995: The minimum qualification to meet Good Delivery standards.

The subsequent numbers denote minor dips in fineness — such as 990 (two nines fine); 986 (ducat fineness); 958.3 (23 karats); 916 (22 karats); and so on. At the bottom of the “fineness” ladder is 333, which denotes eight karats. After 1884, 333 millesimal fineness has been the minimum purity standard for gold in Germany.

How to Measure Gold Purity

When a gold item is bought from a reputed jeweler, the purity or actual gold content of the piece is usually not on the buyer’s mind — for the credibility of the jeweler backs it. Moreover, the gold article would bear a quality mark per the National Gold and Silver Stamping Act of 1906.

But if you acquired the gold from a local jeweler with no trading or sales history, the purity of the piece would be suspect. The aforementioned quality mark or trademark mandated by the law is not that difficult to forge, which some no-name jewelers could resort to as they have no credibility to lose. The dishonesty would hurt the more prominent jewelers big time, and therefore the chances of duplicity happening are pretty slim.

Testing becomes imperative in situations where you are not sure about gold purity or the piece’s exact gold content.

You can find out the purity of gold with various tests. Though not conclusive, the following tests provide a clue or are your starting point for more complex or scientific later tests:

Magnet Test

The simplest and most effective way to test the genuineness of a gold item is a magnet test. Just hold a magnet close to the gold. If it sticks, it’s not pure gold. If the gold is not 24K pure or ferromagnetic metals (such as nickel and iron) are added to the piece, the magnet could attract the gold.

Gold is mainly blended with copper and silver so that the finished product is sturdy or doesn’t bend easily. Both copper and silver are non-magnetic. The two are commonly added to gold (as illustrated in the table earlier). Therefore, the chances of a ready-to-sell, commercial gold product sticking to a magnet are low. If the item sticks to the magnet, it’s most likely not gold.

But in rarer instances when gold is mixed with nickel or iron, it could stick to the magnet and still be considered real gold. White gold usually has gold mixed with nickel, silver, or palladium. The mix ratio would vary with the karat figures. A piece of 18 karat gold, for instance, would be 75 percent gold and 25 percent nickel and zinc.

Float Test

Pure gold has a density of 19.3 g/cm3 (grams per cubic centimeter). Water is 1 g/cm3 dense. In other words, gold sinks when immersed in water. To perform the test, place your gold item in a container or bowl half filled with water. If the piece sinks, it’s likely pure gold.

But there’s a significant flaw with this test.

There are very few objects that are lighter or have a lower density than water. Mahogany wood (0.71 g/cm3), oak wood (0.85 g/cm3), apple (0.96 g/cm3), ice (0.93 g/cm3), etc., are items less dense than water and, therefore, float in water. Some kinds of plastic sail on water too. Other than that, pretty much every other item sinks in water when fully immersed.

And that includes metals too — such as copper (8.96 g/cm3), aluminum (2.70 g/cm3), iron (7.87 g/cm3), alloy metals, etc. Besides gold, other precious metals such as silver (10.5 g/cm3), platinum (21.45 g/cm3), palladium (12.2 g/cm3), etc., also do not float in water.

Therefore, to conclude that the item of gold you float-tested is genuine just because it sunk is imprudent or borderline asinine.

Ceramic Test

Rubbing a gold item onto a piece of unglazed ceramic is another simple, quick way to test gold purity or authenticity.

Carry out the test by pressing the gold item with slight pressure onto the ceramic and sliding it across. If the gold piece leaves behind a golden color streak, it’s pure gold. If the mark is black, the item is not pure gold or has more than just a slight impurity.

Using an Acid Testing Kit

Besides the above tests, you can also employ acids to test the purity of gold. For the experiment, you’ll need a gold testing kit, such as this JSP Gold Testing Kit.

A testing kit would usually comprise multiple bottles of nitric acid and a testing stone, which is also called streak stone or touch stone. The stone is typically black novaculite, a fine abrasive stone that makes leaving behind a trace easier for gold.

The following are the steps to use the testing kit:

  • Put on a pair of gloves, for nitric acid is corrosive and could cause significant burns.
  • Gently rub your gold item (ring, coin, etc.) on the stone. Rub until the gold leaves behind a visible trace on the rock.
  • Pour a few drops of nitric acid on the golden trace and check for a reaction.
  • If there’s no reaction or the gold is still visible through the solution, it’s real gold.

If the acid eats the gold or the marks on the stone disappear a few seconds after the acid drops are poured, it’s safe to assume the gold is a lower karat than the karat the acid solution is designed to be tested on. For example, if the gold trace doesn’t react to a 14K acid solution, it’s likely 18K or more. If the acid completely wiped off the mark, the tested gold is 14 karats or lesser.

If the gold scrutinised is not gold or some other material, the reaction could be slightly dramatic. The solution would bubble, or the gold material would assume a greenish hue after a few drops of the acid solution are poured on it directly. Gold-plated jewelry, such as gold covering sterling silver, would turn milky in appearance.

Safety tip: Ensure the space you’re conducting the test is well-ventilated and devoid of kids and pets. Put on a mask to be on the safer side. The last thing you would want is for yourself or others to inhale the caustic acid.

Testing Measures Vary with the Karat Numbers

The bottles in the acid testing kit are usually labeled with karat numbers they are supposed to work on. The range is generally 10K to 22K. The acid solution designed to test 14K karat gold or more is usually aqua regia (a mix of concentrated hydrochloric acid and nitric acid).

Solutions meant to test the integrity of an 18K gold jewelry should not be used with a higher karat gold or gold jewelry you know is 24K gold. Only an acid solution meant to test 24K gold should be used to derive tangible results.

However, if you suspect a piece of 24 karat gold you purchased is not as pure as claimed or is fewer karats, use the bottle for lower karat gold items to clear doubts. For instance, if the 18K acid solution works on the trace left behind by a piece of 24K gold, you can safely assume the gold item is 18K and not 24K.

And if you are clueless about a gold item’s karat numbers and want to ascertain the same, start the testing with the solution for the lowest karat. You may go up from there until the gold trace changes color.

If you used a 14K testing solution on the gold and the gold marks are still visible through the solution, the gold under the testing is more than 14K. You may continue the testing until the gold reacts or the trace disappears.

P.S. The above instructions are generic or not specific to the testing kit you picked up. Therefore, read testing kit instructions before proceeding.

Karatmeter: A More Scientific Way to Check Gold Purity

The above gold purity testing methods are abrasive, invasive, and also archaic. Electronics can also be employed for a lot more accurate, precise, and non-destructive testing mechanisms. A karatmeter is one such instrument that does that.

Karatmeter is an instrument used to correctly compute the amount of gold in a given piece of gold. You could use it to test the purity or elemental composition of other precious metals too — including platinum and silver.

The machine employs X-rays to provide an accurate purity reading, and it does so quickly (usually under 30 seconds). The device also helps ascertain the other elements present in the not-pure gold.

The Working Mechanism

The karatmeter looks like a scanner machine. The gold article goes inside the device so that it can do its job. While the traditional karatmeter is a stationary machine, you can find modern versions of the device in a handheld or portable form.

The X-ray tech used in a karatmeter is not the same as the ones used in medical applications. Called XRF (X-ray Fluorescence Spectroscopy), the analytical technology or analyzer determines a material’s elemental composition by measuring the secondary or fluorescent X-rays emitted from the sample object after being subjected to the main X-ray source.

Each element existing in the sample – in this case, a piece of gold – generates a set of fluorescent X-rays that are unique enough for the subsequent analysis. The accuracy and precision of the XRF spectroscopy render karatmeters and similar material composition analysing tools ideal for quantitative and qualitative studies.

How is a Karatmeter Superior to the Acid Test

The touch stone method and the karatmeter are based on science — the latter employing significantly more advanced scientific concepts. The thing that sets a karatmeter apart or makes it better than the former is the precision with which it tests elements.

As mentioned earlier, a touch stone testing kit comprises bottled solutions to assess gold that fall in the 10K to 22K range. An 18K testing acid used on a 14K gold item would make clear the examined gold is 14K. However, it only gives an approximate range or does not state the piece’s exact karat value. It could have been 14.5K, 16K, or 17.5K, which the acid test would not be able to divulge. 

A karatmeter asserts the quantity of gold in a given piece – to the karat, to the percentage – and other metal content. For individual testers, knowing the exact gold amount may not be essential. For gold dealers or jewelers, ascertaining the correct gold quantity is critical.

Conclusion

“Gold purity” is not a binary topic, or it’s not as simple as “pure gold”, “impure gold”. There are layers, levels, or grades to consider when measuring gold purity. Hopefully, this article threw light on the various aspects attached to gold purity levels and measuring them.

If you are particular about the gold you acquire, buy it from a reputed jeweler or dealer you know and trust. If a seller is wooing potential buyers with attractive marketing and selling gold at rates much cheaper than the competition, grow skeptical. While you could always test gold for its purity post-purchase, the testing is a step you can avoid if you do your homework.

And if not wanting to test your gold means buying a jewelry piece such as plain gold jewelry, gold chain, a gold engagement ring, etc., at a slight premium, so be it.

FAQs

Does scratching gold against a testing stone diminish its value?

Scratching gold against a stone is a crude way to test the purity or authenticity of a given piece of gold. When rubbed against novaculite, the gold item is indeed letting go, albeit minuscule (micrograms), amounts of gold as traces on the stone.

Some acid testing processes could even entail cutting a tiny portion of a gold item and immersing it in or soaking it with testing acid. In such cases, the amount of gold relinquished for the testing is greater. If you’re wary of the impact rubbing against a touchstone causes on your gold, employ more scientific, non-damaging testing tools.

Do gold jewelers or dealers nefariously embed gold with iridium?

Iridium is a complex, silvery-white, highly dense, corrosion-resistant chemical element employed in special alloys. Unlike traditional gold alloy metals, such as silver and copper, iridium embeds itself into the gold, making it difficult to tell it apart from the yellow metal.

Some jewelers employ iridium in an unethical and illegal (if caught) way. They integrate iridium with gold and pass the item as pure gold. Unfortunately, the aforementioned acid tests will not detect adulteration. Modern karatmeters, however, can make the distinction. That’s one more reason to use a more advanced testing method.

All things said, it’s recommended you buy gold articles from a reputed jeweler or dealer. Fortunately, gold etched with iridium is a non-issue in the U.S., UK, Europe, and most developed regions worldwide. But it’s still advised to exercise caution.

Is the XRF tech perfect?

XRF is, by and large, right on the money (or the metal). But a valid criticism against the technology is it takes up a simple/superficial approach to testing. It measures just the outermost portion of a sample object.

The rays do not penetrate into the core of the metal.

Hopefully, advancements in the field would correct the shortcoming in the future.