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.

Ametrine – A Rare Natural Pairing of Amethyst and Citrine

Ametrine 14k White Gold Pendant Peora

Natural Bolivian Ametrine Pendant

Ametrine, also known as “Trystine” or its trade name “Bolivianite” is a very unique kind of gemstone. It is a rare combination of purple Amethyst and golden Citrine in one stone. Both Amethyst and Citrine are part of the Quartz family of minerals but their distinct rich colors come from different reactions during their growth process.

Quartz is made up of the elements Silicon and Oxygen, but to create the bold colors of the Citrine and Amethyst variations Iron must be present in the compound. Amethyst generally has a smaller amount of Iron than Citrine. For Amethyst to become purple there must be a certain amount of gamma radiation during the growth process to oxidize the Iron from a 3+ to a 4+ state.

This is such a rare occurrence in nature that virtually all of the world’s true natural Ametrine comes from only one mine near Santa Cruz, Bolivia in the Andes mountain range. There are trace amounts of Ametrine found in India and Brazil but they are very light in color and there are very few gem quality specimens. Most of the Ametrine found in India or Brazil are irradiated in a lab to increase color saturation.


Natural Bolivian Ametrine Ring

Today Ametrine has become quite popular and many “look-a-likes” and false Ametrine have entered the market being sold as Ametrine. It is possible to create Ametrine synthetically, but requires a very specific process to get the color sections to come out just right. There are other false “Ametrine” being sold which have very different color combinations that are far from true Ametrine and could not possibly be natural. True Ametrine is always a combination of purple Amethyst and golden Citrine.

Here are a couple of really cool articles that go further in depth about Ametrine if you are interested in finding out more specifics:
Ametrine Information: http://minerals.gps.caltech.edu/Ametrine/Index.html
Tour of Bolivian mine: http://www.gia.edu/gia-news-research-In-Search-of-Ametrine