Diamonds 101: The Four Cs – Carat

“I never worry about diets. The only carrots that interest me are the number of carats in a diamond.” – Mae West

Prince William apparently toted a valuable ring (18 carat oval sapphire surrounded by 14 diamonds) for weeks in his back pack while on holiday in Africa before proposing to Kate.


How big is a carat, anyway?

Carat” is a measure of  weight. One (1) carat weighs 200 milligrams.

The weight of a sapphire, diamond, or other gem is the easiest of the Cs to verify. Simply ask the jeweller to zero his scale and verify the weight in milligrams. Then to convert that back to carats, use this simple formula:

Divide the actual weight in milligrams by 200 to get the number of carats.

Example:  Say the actual weight is 450 mg.    450 ÷ 200 = 2.25 carats.

A “point” is one-hundredth of a carat. Jewellers often describe the weight of a gem that is less than one carat in terms of points. One carat is subdivided into 100 points. A ¾ carat diamond would be ¾ x 100 = 75, or a “75 point” diamond. In milligrams it would weigh ¾ of 200 mg, which is 150 mg.

To verify that the point size is accurate, ask the jeweller to weigh the diamond. Divide the point size by 100 and multiply by 200 to get the equivalent weight in milligrams.

Example: The salesperson says the diamond is 63 points. 63 ÷ 100 = 0.63. The scale weight should be 0.63 x 200 = 126 mg. If the actual weight is less than 126 mg, then the real point size is smaller.

A diamond that is under one carat, even one point less than a full carat, is considerably less expensive. Visually there is not much difference, so those on a budget may wish to ask the jeweller to show them diamonds in the range of 90-99 points.

To make things complicated, the size of a diamond is not correlated with the weight. According to, two diamonds of the same carat weight may appear to be different sizes depending on how the diamond is cut. Some diamonds will have extra weight on the bottom part — or pavilion — of the stone, and therefore appear smaller. See my October 3 post for a description of “Cut”.

I’ve covered two of the essential Cs – Cut and Carat. Up next is “Clarity”.

Bigger is not necessarily better!

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