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Carat Weight
One of the Four C's for evaluating diamonds
by Gary Dutton


This is the easiest "C" to understand. The weight of a diamond is directly related to its size, and, although incorrect, these terms are sometimes used interchangeably. The unit of weight for diamonds and other gemstones is called the carat. This is a term which comes from the ancient use of the very uniformly-sized carob seed in measuring small weights. A table of common weight-related terms is given below.

Carat Weight Equivalent Weight
One Carat

200 milligrams = 0.200 grams (There are 5 carats in 1 gram and 142 carats in 1 ounce.)

A carat is divided into 100 parts each called a point. So 1 carat = 100 points.

Three-Quarter Carat 75 points
Half Carat 50 points
Quarter Carat 25 points
One Grain or "a one grainer" 25 points or 1/4ct.; A diamond of approximately 1ct. would be called a 4 grainer, one ~ 1.5ct. would be a 6 grainer, and a 0.5ct. stone a 2 grainer.
Melee Diamonds weighing less than 1/5ct. (less than 20 points). Stones this size are set using various techniques as groupings in jewelry.


The relationship between how large a diamond looks and its actual weight can be deceptive. For well-proportioned, round diamonds the size (diameter)-weight relationships are given below (The diamond images are not actual sizes, and their proportional differences are only approximate.). As examples, a 1.00ct. stone should normally be about 6.5mm in diameter, a 3.00ct. would be 9.3mm, etc.


However, if a 1.00ct. round diamond has a shallow cut, sometimes called a "spread cut", it will be will look larger than a well-cut stone of the same weight because its diameter will be larger than 6.5mm. You might think that's great, but the trade off is that the spread cut diamond will often have a lifeless, glassy look, called a "fisheye".

Conversely, when a 1.00ct. round diamond is cut too deep, it appears to be of smaller size than a well-cut stone, because the diameter will be smaller than the expected 6.5mm. This means that you are paying for extra weight in the pavilion and girdle areas, which doesn't add to the beauty of the diamond. Furthermore, the stone will often appear dark and dull in the center, resulting in what's called a "nailhead".

The take-home lesson is that it is very important to consider only a well-cut diamond, even, if necessary, at the expense of the other 3C's (carat, color and clarity). Superior cut will give you superior light performance producing a bright and lively stone. It may not be the biggest looking diamond on the block, but it will be the BEST looking diamond!

The same arguments concerning shallow and deep cut stones also hold for the other fancy shaped diamonds, although the proportions will be different. You can read about this subject in our article on fancy shapes.

A final word about weight representation. In the U.S., the Federal Trade Commission requires that diamonds be weighed to one-thousandth of one carat (0.001ct.), but that the final weight can be rounded to the nearest half point (0.005ct.). This means that a diamond weighing 0.995ct. can be represented to you as a 1.00ct. stone. However, international convention is more strict, and a weight can only be rounded to the next higher point (0.01ct.) if the actual weight reaches 9/10ths (0.009ct.) of a point. For example, the 0.995ct. diamond must be represented as a 0.99ct. stone, and to be represented as a 1.00ct. diamond the stone must weight 0.999ct. before it can be rounded up to 1.00ct.

The GIA follows the more strict international convention in listing the weights in their grading reports. AGS gives the actual weight directly to 1/1000ct. in their grading reports. Dutton's Diamonds follows and supports the more strict international convention in carat weight representation.

Effect of carat weight on price: Because diamonds are more rare the larger they are, the carat weight is not directly proportional to the total asking price. For example, the total price of a 2ct., D, IF, stone is not twice the total price of an identical 1ct. stone, but is more than three times the price, and an identically graded 3ct. diamond is about 7.7-times as expensive as the 1ct. stone. Also, a premium is added to diamond prices as they reach and exceed each 1/4ct. increment in weight. (Hint: One way you might save a little money is by buying stones with weights just below those increments, if you can find them.)

Color    Clarity   Carat Weight   Cut

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Gary Dutton, Ph.D., G.G.
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