Diamond Carat Weight

August 2013

The standard unit of measurement for diamonds and other precious gemstones is the carat. This should not be confused with karat, as in 14K gold, which signifies gold purity. Since 1913, the metric system has been used to gauge the carat weight of diamonds with one carat equaling 0.2 grams.

Carat Weight and Price

Diamonds are mined from deep within the Earth's crust. Most rough (uncut) diamonds are generally either too small or of such poor quality that they are unable to produce the sparkling, lustrous diamonds which the world jewelry market demands. For example, it might be necessary to mine up to one million rough diamonds just to produce one single, one-carat diamond.

Because of the rarity of larger, higher-quality diamonds, the price differential can be enormous among various carat weights. It is possible for a one-carat diamond to be four to six times more expensive than a half-carat diamond with the same grades of clarity, color, and cut.

A carat is divided into 100 points. Therefore, a 25-point diamond would have a carat weight of 0.25. It is important to note that 2 diamonds of the same carat weight can have very different values attached to them. This is due to the fact that the other members of the Four Cs, clarity, color, and cut, also affect the overall value (price) of a diamond. Most diamonds used in fine jewelry have a weight of one carat or less.

The Origin of the Carat

The meaning of the word carat comes from the Mediterranean region where carob trees have grown and flourished for thousands of years. Remarkably, carob seeds are nearly identical in size and for this reason these tiny seeds were commonly used as units of measure by early diamond merchants. One carob seed is the approximate weight of one modern carat.

Interestingly, in the Far East grains of rice were historically used to measure diamonds. From this ancient practice comes the word "grainer" which is still in use today. Gemologists still refer to the weight of diamonds in "grainers" with a four-grainer diamond equal to one carat.