When shopping for gemstone jewelry—especially engagement rings—”Karat” and “Carat” are two frequently used terms that sound exactly the same. However, despite their identical pronunciation, these words carry entirely different meanings and measurement standards in the context of jewelry. This phonetic similarity is the primary reason why many consumers confuse the two, and failing to distinguish them correctly can lead to misunderstandings about a piece’s value, quality, or suitability. This article aims to break down these two key concepts, trace their historical origins, and explain their impact on jewelry value—helping you become more knowledgeable about your jewelry choices.
What Is Karat?
Is “Karat” a unit used to measure diamond weight? Actually, no. You might feel a little confused when you see this term by itself, but what if I add a number in front—like 18 Karat? Sounds familiar now? And if I use its abbreviation, 18K, you probably know exactly what it means.
Karat (abbreviated as K or kt) is a unit that measures the purity of gold, indicating how much pure gold is present in an alloy. It’s based on a 24-part scale, where each karat equals 1/24 of the whole, or approximately 4.17% pure gold. For example, 18K gold consists of 18 parts pure gold and 6 parts other metals, making it a 75% pure gold alloy.
The History of Karat
The term “Karat” originates from the ancient practice of using carob tree seeds as a standardized unit of weight in trade. In the Middle Ages, a coin called the “mark” weighed 24 carats (in this context, “carat” shares the same root as the gemstone weight unit and was theoretically equal to the weight of a carob seed). Since pure gold is too soft to be used for minting coins, copper or other metals were added to form a harder alloy. The purity of these coins was expressed by the proportion of pure gold within their total carat weight—this is the foundation of the modern Karat system used for gold today.
The Impact of Karat on Jewelry
Gold is typically graded in Karats such as 24K, 18K, 14K, and 10K. As the Karat number increases, the purity of the gold rises, but its hardness and durability decrease. The higher the Karat, the more expensive the gold tends to be. (For instance, 18K gold is softer than 14K gold but more expensive.)
From the perspective of daily wear, higher Karat doesn’t necessarily mean better. For example, 24K gold is pure gold, making it the most expensive but also the softest, which means it can be easily scratched during regular use. That’s why we generally don’t recommend 24K gold for everyday jewelry. However, if you’re buying gold as an investment, that’s a different matter.
Check out our Gold Karat Guide to learn more about K gold.
What Is Carat?
This “Carat” is the one used today to measure the weight of diamonds, and it’s a key component of the diamond 4Cs. Carat (abbreviated as ct) is a unit of measurement used to describe the weight of gemstones—particularly diamonds—not their size. One carat is defined as 200 milligrams, or 0.2 grams. For gemstones weighing less than one carat, the carat is further divided into 100 points. So, a 0.75-carat gemstone would be referred to as 75 points.
The History of Carat
The term “Carat” shares the same origin as “Karat,” both stemming from the early practice of gemstone traders using carob seeds as counterweights on balance scales, since these seeds were believed to be remarkably uniform in weight. This term has been used since the 16th century and has become the globally recognized standard for measuring gemstone weight.
The Impact of Carat on Jewelry Value
Carat weight significantly affects the value of a gemstone—the higher the carat weight, the higher the value, assuming other quality factors are equal. This is especially true for diamonds, where the price often increases exponentially rather than linearly. Check out our Diamond Guide to learn more.
It’s important to note that carat refers to weight, not size. Although carat measures how heavy a diamond is, two diamonds of the same carat weight can look very different in size depending on their cut and shape. For example, a 1-carat round diamond can be cut to show a 5×5 mm surface or a 6×6 mm surface, depending on the cut depth. This means you can achieve a larger visual appearance without increasing the carat weight by choosing the right cut and shape.
Karat vs. Carat: Side-by-Side Comparison
To help clarify the distinction between these two terms, we’ve created a side-by-side comparison chart highlighting their core differences:
| Term | Karat (K or kt) | Carat (ct) |
|---|---|---|
| What It Measures | Gold purity | Gemstone weight |
| Unit | 24-part scale (1/24) | 200 milligrams (100 points) |
| Key Takeaway | Higher karat = higher gold content, higher value, but softer | Higher carat = heavier gemstone, but cut/shape/4Cs determine appearance and value |
Clearing Up Common Misconceptions
“24-Karat Diamond” Is Incorrect
It’s important to point out that saying “24-karat diamond” is a common mistake. Diamonds are measured by weight in carats, not by gold purity in karats. So, there is no such thing as a “24-karat diamond” in terms of purity.
Regional Terminology Differences
In the U.S., “Karat” is used to describe gold purity. However, in some countries, such as Australia and the U.K., “Carat” is also used to refer to gold purity. This creates confusion globally. In such contexts, the two terms may be used interchangeably for gold, but “Karat” is never used to describe gemstone weight. This “contextual interchangeability” highlights the importance of geographic context in jewelry terminology—something consumers should be especially mindful of when purchasing internationally.
Gemstone Purity vs. Weight
To reiterate: for gemstones, “carat” refers to weight. A gemstone’s “purity” is instead related to clarity and color, not its weight. A heavier diamond doesn’t necessarily mean it’s more “pure” or clearer—it can still contain inclusions or flaws.