carat

You've probably heard about carats when talking about diamonds—it's how we measure their weight (which often, but not always, affects how large they look). But this unit isn't just for diamonds! The term is traditionally linked to the seeds of the carob tree, which were used as handy reference weights in trade. Today, one metric carat is standardized at exactly 200 milligrams.

But hold on—there's more! In many places, a very similar word is used to measure the purity of gold (often spelled "karat" in the US). "24-karat gold" means essentially pure gold, while lower numbers indicate a blend of gold and other metals. When you hear "18-karat gold," it means 18 parts out of 24 are gold, and the remaining 6 parts are other metals added for strength and color.

The word "carat" has two primary meanings. One is a standardized unit of weight for gemstones. The name is traditionally linked to the carob tree: people have long told the story of traders using carob seeds as convenient reference weights. Today, one metric carat is strictly defined as 200 milligrams (0.2 grams). When discussing gemstones like diamonds, sapphires, or rubies, "carat" refers to the stone's weight. While higher carat weights often correspond to a larger-looking stone and a higher price, the same carat weight can look different depending on the gem’s cut, proportions, and density.

The other meaning appears in the spelling "karat" (often abbreviated as "K" or "kt"), which is used to measure the purity of gold. In this system, pure gold is 24 karats, meaning 24 out of 24 parts are gold. Because pure gold is very soft and malleable, it is usually alloyed (mixed) with other metals like copper or silver to increase its durability for jewelry. An 18-karat gold ring, for example, contains 18 parts gold and 6 parts other metals. In British English, the spelling "carat" is often used for both weight and purity, whereas American English typically distinguishes between "carat" (gem weight) and "karat" (gold purity).

Whether you're admiring the sparkle of a diamond or choosing a gold wedding band, understanding the "carat" helps you appreciate both weight and purity—the two key factors in determining a treasure's value and quality.