What is a carat? Carat Weight and Millimeter Dimensions of a Gemstone Explained

We get quite a few inquiries from customers asking why the same millimeter size of gemstone has different carat weights from one gemstone to another. We also get customers saying he/she expected the gemstone to be larger/smaller because of the carat weight listed. We thought we should take a few minutes to explain why this is.

Aquamarine Gemstone Earrings - Peora JewelryCarat is a unit of measurement. It is used to measure mass or weight. You might think of diamonds when you hear the word “carat,” but it is actually used to measure all types of gemstones in jewelry.

A carat is exactly 200 milligrams or 0.2 grams. When referring to diamonds, most jewelers will use the term “point” or “pointer” to describe the size of a specimen. As you may have guessed, a “point” is 0.01 carat or 1/100th of a carat. So a ten “pointer” diamond would be 1/10th of a carat or 0.1 carat.

Now, when we try to equate carat weight to physical dimensions like millimeters, things get a bit tricky. Because diamonds and gemstones come in all sorts of shapes, each shape will convert differently between carats and millimeter size. With diamonds, it is generally accepted that a 1.00 carat round diamond will be 6.5 mm in diameter, but a 1.00 carat princess or square shape diamond will be 5.5 mm in diameter.

Emerald Gemstone Pendant Necklace - Peora JewelryDiamonds are more or less the standard comparison for carat weight and millimeter size. However, depending on the type of gemstone, the carats to millimeter difference can vary widely. Each gemstone or mineral type has a different density, which means it will convert from carats to millimeters differently.

Let’s take the diamond’s most commonly used inexpensive stand-in, Cubic Zirconia, as an example. The Cubic Zirconia is actually much denser than diamond, roughly 1.7 times as dense. So a 1.00 carat round CZ would actually be only a 5.3 mm diameter compared to the 6.5 mm of a diamond with the same carat weight. Some other gemstones that are more dense than diamonds include: sapphire, ruby, alexandrite and garnet.

Garnet Gemstone Ring - Peora JewelryConversely, let’s take Opal as another example. Opal is significantly less dense than diamond so it will be a much larger size stone with the same carat weight. A 1.00 carat round Opal will be almost 8 mm in diameter compared to the 6.5 mm 1.00 carat diamond. Other gemstones that are less dense than diamonds include: amethyst, tourmaline, aquamarine, emerald and quartz. Topaz and Peridot are fairly close in density to diamond.

So what do we take away from all of this? Basically, if you are searching online for a piece of gemstone jewelry that is not a diamond, you might as well disregard the carat weight altogether. Just look for the millimeter dimensions of the gem and pull out your trusty ruler.


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