The 4 C’s - Diamond Carat

Of the commonly referred to “4 C’s” of diamond quality, “carat” is one of the first things mentioned when talking about diamonds. While it is the most basic physical attribute of a diamond, there are some important things that you should know to fully understand the first “C”- diamond carat.

Diamond Carat Weight

Carat is a measure of WEIGHT. The weight of a diamond is expressed in carats. This term comes from ancient times when gems were weighed against the carob bean. One carat is equal to 200 milligrams, or one fifth of a gram.
Diamond Carat Weight
While there is a general correlation of weight to size, diamonds with the same carat weight can vary greatly in terms of their actual dimensions. Because diamonds are sold by the carat, diamond cutters tend to want to retain as much weight as possible from the rough they buy. By proportioning diamonds for maximum carat yield, many diamonds are cut deeper or thicker at the girdle. This results in diamonds that are smaller looking than their carat weight would indicate and commonly suffer from significant light performance deficits as well. In other cases, a piece of rough might be able to yield a diamond with larger dimensions by cutting it shallow. This will also result in a diamond that will have weak light return. The weight to size ratio of a diamond is referred to as “spread”. Proper cutting is essential to diamond light performance, sparkle and beauty, and weight ratio (spread) is another indication of diamond cut quality.

Points and Grains

Carat weight is sometimes expressed in “points”. These are percentage points of a carat where one point equals 1/100th of a carat, so 25 points is equal to a quarter of carat and 50 points is equal to half of a carat. In early times diamond weight was also compared to grains of rice. Four grains of rice were equal in weight to a carob bean. Diamond dealers still use the term when referring to general sizes such as “three grainer” for diamonds in the .70-.79 ct range, or “five grainer” for diamonds in the one and a quarter carat range.
Diamond Carat Points

Carat Weight and Cost

As carat weight gets larger, the value of the diamond increases disproportionately. This is because pieces of rough material are increasingly rare with size. Over a million pieces of rough must be mined to find one piece large enough to produce a 1 carat finished diamond. The result is that a diamond twice as large as a smaller one of the same quality may be three or more times as expensive, depending on the specifics of the market. Diamond prices are determined by market forces of supply and demand.
Cost, which is sometimes referred to as the 5th C, is to some extent driven by rarity factors that don’t impact diamond beauty to any meaningful extent. Please see these helpful guides on the effects of diamond clarity and diamond color grades on diamond beauty.

Diamond Magic Marks

Prices per carat increase at certain size thresholds, sometimes referred to as “magic marks”. For example, the price per carat is higher for a 1.00 F VS2 diamond than .90 F VS2. The compounding effect on total price of a lower price per carat and slightly less carat weight can result in a significant savings in total price, even though the visual size might be very close. If the diamond of your dreams is above your intended budget, you may want to think about purchasing a diamond of the same color and clarity just under the threshold of the weight that you have in mind. Of course, diamond manufacturers are very conscious of market valuations, so most diamonds are produced to finish at or just above such thresholds. The market also tends to adjust for the increased demand in these areas, particularly in ideal cut diamonds.

Total Carat Weight

Combined diamond weight in a finished jewelry piece is referred to as ‘Carat Total Weight’ and abbreviated ‘’ For example, a three stone diamond ring set with three diamonds, each weighing 1/3 carat, weighs approximately a total of one carat or 1.00 ct. tw. Sometimes you will also see jewelry pieces, often containing colored gemstones, stated “total gem weight”.

Carat Weight and Cut

As stated above carat weight is not the same thing as size, although there is correlation. It is cut craftsmanship that can impact significantly both the actual dimensions and apparent size of the diamond. Diamonds cut too shallow may have a larger spread than it should, while diamonds cut too deep have a smaller spread. Too shallow and too deep are both undesirable.
Diamond Carat Weight and Cost
A superior cut will result in light being returned to the eye from the entire top of the diamond. This “edge-to-edge” light performance will enable the diamond to appear larger than diamonds of the same size that are not cut as well. In the example below both diamonds are the same size but the ideal cut diamond on the left looks larger because it is returning light from edge to edge.
Pair of Round Cut Diamonds
(Image courtesy of Garry Holloway)
The most important thing to remember about the diamond 4C’s is that while carat is a value factor, Cut is King!

Carat Weight to Millimeter Size Conversion

As stated above the relationship between carat weight and actual dimension varies according to the cut quality of the diamond. The chart below provides an approximate conversion for well-cut round diamonds.
Diamond Carat to Millimeter Chart

Carat or Karat?

The term karat has a slightly different meaning. It too came from the carob seed, but became a variant dealing strictly with gold. Its definition states that a karat is “a unit for measuring the fineness of gold, pure gold being 24 karats fine”. Pure gold is extremely soft and when used in jewelry can easily be damaged. Therefore, copper, or another metal alloy is mixed with the gold. 24 karat gold is completely pure, while 18 karat gold will have 18 parts gold and the other six parts will be the other alloy (75% pure). 14 karat gold would be 14 parts gold and 10 parts alloy, and so on. In the United States of America, karat markings are always designated with a “K”, never a “C”. Stamping of gold purity on jewelry items is regulated by law.
These distinctions between carat and karat should help to clear up any confusion of the terms. This knowledge is definitely essential when making any significant jewelry purchases.