While many confuse carat as a measurement of physical size, it is actually a measurement of weight used to describe diamonds and other gemstones. One carat equals 0.2 grams, about the same weight as a paperclip.  

To understand what diamond carat measures, it’s helpful to know the origins of the carat system. It began with carob seeds that early gem traders used as small, uniform counterweights to their balance scales. At that time, gemstone measurement varied internationally; there was no standard. It wasn’t until 1913 that the world adopted the modern metric carat. Now, a carat weighs the same in every corner of the globe.  

As one of the four factors that describe and determine a diamond’s quality and value, carat should not be confused with ‘karat,’ a unit used to measure the purity of gold.

No, diamond carat and diamond size are not the same thing. Carat is a metric unit that measures weight — not size. Equating the two may seem logical — but it isn’t. It’s best not to use carat weight alone to estimate a diamond’s size. 

For example, different gemstones have different densities (mass per unit of volume), meaning that two gems with the same carat weight may exhibit very different sizes. The cut of a gemstone can also affect its carat weight. If a round diamond is poorly cut, it may be too deep or have an extra thick girdle. If excess weight is hidden around the girdle, it won’t display once set, which means that a 1.20-carat diamond with a ‘hidden’ weight may look the same size as a 1-carat diamond. So, a customer might pay for the added weight with no visual benefit.

As mentioned, carat weight has a significant impact on price. Larger diamonds are much more valuable because they are discovered in nature less frequently than small diamonds. This means that diamonds are not priced linearly — a one-carat diamond will cost more than two half-carat diamonds of the same quality. 

Carat’s impact on price is clearly displayed when considering what the diamond industry refers to as ‘magic sizes.’ At certain weight boundaries (a ½ carat, ¾ carat, and 1 carat), or ‘magic sizes,’ the price per carat increases dramatically. These set boundaries coincide with popular weights, which means greater demand and even greater prices. For example, if one diamond weighs 0.97 carats and another of the same cut, color, and clarity weighs 1.03 carats, you probably wouldn’t be able to tell the difference between the two. However, the cost difference will be significant because the second diamond is over the ‘magic’ threshold of one carat.