Diamond Carat Weight

Carat Weight Basics

The Objective "C"

Carat Weight is the simplest and only truly objective factor of the 4C's. Diamond carat weight is also the biggest influence on price followed by the other 4Cs of Diamond Clarity, Diamond Color, and Diamond Cut.


71.73ct Lesotho One Diamond
Photo: Sotheby's
The Lesotho One - 71.73 carat emerald cut diamond

The original rough diamond named "The Lesotho" was over 601 carats, and was purchased by Harry Winston in 1968. The rough was cut into 18 gems, and the Lesotho One was the largest at 71.73 carats.


What is a "Carat?"

1 carat = 200 milligrams or .200gm

1 carat = 100 points - so a point is 1/100 of a carat

Sometimes you may hear a jeweler refer to a diamond in points. A 25 point (25pt) diamond is a .25ct or 1/4ct.

*Fun Fact - The origin of the word "carat" comes from the carob seed. Gemstones were once weighed on balance scales, and gem dealers used carob seeds as counterweights, because of their uniform shape, size and roughly equivalent mass. Carat weight was standardized to .20gm in the early 20th century.


Carat Weight Vs. Size

The two diamonds pictured below illustrate that weight does not equal size. Both gems are the same diameter, but the well cut diamond on the left weighs .84ct and the overly deep cut diamond on the right weighs 1.00ct. Both stones could be cut from the same piece of rough shown between them. The deep cut 1.00ct will most likely sell for a higher price, because it is heavier. Round diamonds with excessive depth appear smaller and show less brilliance and life than stones that are well cut.


Well cut diamond vs. poorly cut diamond
Carat Weight does not equal size


*Note - While excess depth is undesirable in round diamonds, depth is function of design with some Fancy Shape Diamonds, which are cut to different specifications.


Carat Weight and Value

When all other factors are equal, carat weight will have the greatest impact on diamond prices. A 1.2ct G VS1 will cost more than a 1.0ct G VS1, but a 1.0ct D IF will cost more than a 1.5ct K SI1. Remember different combinations of the 4Cs affect price, but carat weight trumps all when comparing similar quality.


Carat Weight - The Nitty Gritty

"Magic" Weights

Diamond price increases exponentially at certain weights. You will see prices jump at .50ct, .75ct, and 1.00ct etc. The 1.00ct weight is particularly popular, and it is economically tempting for a cutter to leave a little more weight on the crown, pavilion, and the girdle to push the stone to 1.00ct or more. Savvy shoppers will search for diamonds just under these magic weights to get a little more for their money, but these diamonds are often harder to find.

The chart below shows that diamond cutters are successful at achieving magic carat weights, and it also reflects consumer demand.

Carat weight availability distribution chart
Carat Weight does not equal size


International Carat Weight Rounding

International weight rounding is not the same as what you learned in school. Diamond weights are rounded up from the third decimal point only when it is a nine.

0.995ct = 0.99ct
0.998ct = 0.99ct
0.999ct = 1.00ct

Estimated Carat Weight for Mounted Diamonds

Diamonds set in a piece of jewelry cannot be weighed, so carat weights are estimated by measuring the outside dimensions of the stone. This is common for an appraisal, which is a report that states the value of a finished article. Such an appraisal should state "estimated" or "approximate" weight, and a margin of error of about 5% is common.

Total Carat Weight

Remember to pay attention to total carat weight when considering mounted diamonds in finished jewelry. A diamond halo ring may have a .75 carat center stone with .25 carats in small surrounding diamonds, but may be advertised as 1ctw or 1 carat total weight. Pay attention to the carat weight abbreviation, and make sure you ask any pertinent questions related to the carat weight of a jewelry piece.

Ring Size and Carat Weight

Wondering what size/weight to buy? The chart below illustrates general diamond size in mm per carat weight in relation to finger size. The chart represents round diamonds. Fancy Shapes will have varying spread, and should be evaluated individually.

Carat weight and percentage of finger coverage per ring size
Relative size on finger in relation to carat weight


Carat Weight and Spread

The girdle or edge thickness is also important. If a diamond has a very thin girdle, the stone can have a better spread, but it can chip more easily. Medium to slightly thick girdles are safest, but thicker girdles add extra weight or reduce the spread.


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