How are Lab-Grown Diamonds Sized? Millimeters or Carats?

Lab-grown diamonds and mined diamonds are sized the exact same way — in both carats and millimeters. Carats get all the glory when talking about diamond sizing, and most consumers have an ideal carat size in mind when they go to buy a diamond*. This is largely due to social pressures and long-held popular beliefs that the bigger the diamond, the more the love. But actually, it’s very important to understand how millimeters come into play. The reason is that carats measure the weight of the diamond, not the size. So sometimes, depending on the cut, a lower carat diamond can actually look larger than a higher carat diamond! When you browse our collection of lab-grown diamonds, you’ll notice we post the carat size, as well as the measurements for length, width, and depth (in millimeters). In this blog, we’ll explain why.

*Reminder: when determining the quality of a diamond, size is only a small portion! The Four C’s of diamond quality include cut, color, clarity, and carats. Learn more about our certified made diamonds here >

What are carats and millimeters?

A carat (ct.) is the unit of measurement used to weigh diamonds. The origin of the word is pretty fascinating. Way back when before there were scales and units of mass, diamond traders compared the weight of diamonds to the weight of seeds from a commonly found tree, the Carob tree (which, fun fact, bears fruit that is a substitute for chocolate!). These seeds were unique in that they each had a uniform weight of 0.2 grams (or 200 milligrams). Today, it is still true that in mined or made diamonds, a carat is equal to 0.2 grams (source). This is not to be confused with the word karat, which measures the purity of gold. As you probably know, pure gold is 24 karats.

Millimeters (mm), on the other hand, give a more accurate depiction of how a diamond will look when worn. Most diamonds offer mm measures of the length, depth, and width of the stone. Knowing the mm measurement is also important if you’re buying a loose diamond for an existing ring setting.

What is Depth Percentage?

There is one exception to point out when it comes to diamond sizing, and that is depth percentage (depicted as depth %). Each of our created diamonds has facts about its weight in carats and its size in millimeters, but depth percentage is something different. This measures how much light is reflected back to an observer, and it’s calculated by dividing the depth of the stone by the width of the stone. If you want your lab-grown diamond to look as large as possible, you should opt for depth percentage that is on the lower end of the ideal range. In round stones, for example, the ideal range is  56.5 and 65 percent, so you’d want to choose a made diamond that is closer to the 56% depth percentage mark for it to appear as large as possible.

We know that buying a created diamond online is different from going to a store to try on and purchase a mined diamond. We’re here to help every step of the way. Contact us any time to talk about lab-grown diamonds and determine what’s best for your unique needs so that you can create your moment!