Understanding Carat Weights – And Why We Typically Refer to Millimeters Instead of Carats

You'll notice most jewelry stores talk about engagement ring stones in carats. That's because the traditional jewelry industry assumes you're buying a diamond. Since we work with natural diamonds, lab-grown diamonds, moissanite, sapphires, and other gemstones, we find millimeter measurements much more useful.

Here's why: carat measures weight, not size. Different stones have different densities, and they're also cut differently based on their crystal properties. Sapphires are typically cut taller than diamonds, and oval moissanites are cut much taller than oval diamonds. This means a 1-carat sapphire looks different than a 1-carat diamond, even before you factor in cut variations. When you look down at your hand, what you see is the dimensions of the stone, not its weight. Millimeters tell you what you'll actually see and what will actually fit in a particular setting.

The chart below compares typical carat weights to millimeter sizes for different stones. You can print out a PDF of this chart to see the actual size of these different stone sizes and how they might look on your hand (be sure to print at 100%).

Something else to consider when choosing a stone size is the proportions. In the illustration above, the rings are pictured in a US ring size 7 and have a 1.5x1.5mm band. But the dimensions of the different components of a ring will affect how big or small a stone will appear, as you can see below in the illustration of a round 5mm stone size.

Below are diagrams that compare some of our popular styles with different stone sizes.