Gold and carats: 24, 18… what does it mean?

We’re launching our blog with a fascinating topic: gold and carats. You’ve probably seen terms like “24-carat gold,” or 14K, 18K… even 750 ml gold. But what does each of these mean? Well, it’s actually quite simple. Let’s dive in!

Gold is a noble metal that reacts with very few other elements. This is one of the reasons why it is so valuable: it doesn’t oxidize, it doesn’t lose its shine, it’s easy to work with yet very strong… However, it has one small issue: it’s soft. Yes, pure gold is (relatively) soft. You’ve probably seen pirate movies where, upon finding an old gold coin, the characters bite into it—supposedly to check if it’s real gold.
Why do they do that? Because gold is so soft that, when pressed between the teeth, it leaves a mark. But be careful! This only happens with pure gold. When gold is mixed with other metals (for example, it was sometimes mixed with lead, which was much cheaper), the resulting alloy becomes much harder.

Precisely to determine the purity of gold, the term “carats” was coined in medieval Europe. It comes from the Italian “carato,” which in turn comes from the Arabic “qīrāṭ,” which originates from the Greek “kerátion,” meaning “carob seed.” This term eventually became a unit of weight measurement. Yes, these are just fun facts—but fascinating ones!

The fact is that “carat” began to be used as a unit to measure the purity of gold within an alloy, based on a scale of 24 parts. This means that the number of carats indicates the amount of gold in an alloy. So, 24-carat gold is pure gold: 24 parts out of 24. It is also called “fine gold,” and it has a purity of around 99% (since achieving a higher level of purity is quite expensive). If an even higher purity is required (up to 99.9999%), the gold is certified and is not used for crafting but rather as an investment.

From there, the most common type of gold—so common that it is often referred to as “hallmarked gold” or “first law gold” in Spain—is 18-carat gold. This means that out of 24 parts, 18 are gold, or in other words, 75% of the alloy is pure gold. This is why you might see it labeled as “750 ml gold“, which stands for 750 thousandths (750 parts gold per 1,000). Jewelry is typically made with this type of gold for two reasons: the first, of course, is that the price of the piece is slightly lower (gold is very expensive, and we’ll talk about that in other posts). But above all, it is because, when mixed with other metals (silver, copper… there are many alloys), gold becomes harder, making jewelry more durable. Can handcrafted jewelry be made with 24-carat gold? Of course! But after wearing it a few times, it will start showing marks—especially if it’s a ring.

From there, gold can be found in any carat value, although the most common ones are 14-carat gold and 9-carat gold. In these cases, the goal is indeed to obtain a more affordable metal: both 14K and 9K gold are still yellow, shiny, beautiful, and very durable, but their price is significantly lower.

How can you determine the carat value of gold, for example, in a piece of jewelry?
There are several methods, such as spectrometers (which use X-rays) or even electronic testers, which measure the conductivity of gold. However, the most common method is the acid test. This test involves a set of acids that react with the gold and a touchstone where the metal is scratched.

The process is very simple: you scratch the jewelry slightly against the touchstone until it leaves some marks. Then, a drop of acid is applied to those marks, and each acid “dissolves” a different level of carats. If, when applying a drop of 24-carat acid, the mark disappears, it means the gold has fewer carats. The process continues by applying drops of 18, 14, and so on, until one mark remains intact—at that point, you know that the gold has at least that carat value.

And remember: at Sensei Jewels, we handcraft our jewelry, which means we can work with all the varieties mentioned above. We typically make our pieces in sterling silver or 18-carat gold, but we always offer the option to create them in 14K or 9K gold (the two most common alternatives). Or in white gold, rose gold… whatever you prefer!