How Big is a Carat?

An ideal cut round brilliant diamond measuring 6.5mm is equal to 1 carat. This fact might lead you to believe that the same is true for rubies, tourmaline, and everything in between. Not so! Carat weight can be a tricky concept to understand when considering different types of gemstones, but we’re hopeful we can clear up several of the common misconceptions surrounding the topic.


Carat weight is one of the Four C’s of Gemstones, alongside cut, color, and clarity. To be precise, one metric carat weighs in at 200 milligrams or one-fifth of a gram. Carat weight is often expressed on a point system, which can be easily understood by thinking of pennies on the dollar. A 1 carat stone is equal to a 1.00 point stone. A half carat stone is equal to .5 points.

Gemstones can vary widely in weight based on the type of gem you are measuring, so the density of the stone is expressed through a specific gravity rating. Cinnabar, for example, is one of the heaviest gemstones, averaging a specific gravity of 8.0-8.2. Spessartine Garnet has a specific gravity of 4.12, and for comparison, a diamond has a specific gravity of 3.5. Amber is one of the lightest weighing stone, with a specific gravity of 1.05. It would take a larger specimen of amber to add up to a 1-carat total weight, compared to a spessartine garnet. Opal is another lighter weighing gemstone, with a specific gravity of 1.88-2.5. Amethyst has a specific gravity of 2.65, meaning that a 1-carat amethyst would look much larger than a 1-carat diamond.

To illustrate the example of specific gravity and how that relates to the appearance of a gemstone, imagine a 6.5 mm round diamond. This is equal to a 1-carat diamond. A ruby or sapphire with the same dimension would have a total carat weight of 1.15. Inversely, a 6.5 mm opal would weigh .65 carat because of the opal’s lower specific gravity.

gemstone anatomy

The carat weight of a gemstone seems like a straightforward topic, however, the carat weight and the desired appearance of a stone depend on the cut. The cut has the most dramatic effect on the way the stone appears in a mounting. If a stone has a shallower cut, it will “face-up” much larger due to a wider table on the gemstone. If a stone is cut deeper, it will likely look smaller from the top of the stone. In that case, the majority of the carat weight is held below the table of the stone in the pavillion.

Bear these facts in mind when looking to purchase your next gemstone. Considering factors such as the gemstone’s cut and specific gravity while eyeing the carat will make you a much more informed buyer, and in turn, a happy customer!

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