Carat weight and value

Diamond weighs are expressed in metric measurements called carats (ct). One carat is equal to 200 milligrams (1/5 of a gram, or 0.200 gram).

In ancient times digital scales did not exist, so the carat system sprouted from a crab seed which is fairly uniform in size and weight. Even when more precise weighting methods evolved, carat weight wasn’t constant: a "one-carat" diamond might have weighed anywhere from 0.95 to 1.07 metric carats. A carat was standardized in the early 20th century. Nowadays, the carat stands for the same gem weight in every corner of the world.

A diamond's price depends on lots of factors and the weight is one of them. A 1.00 carat diamond may be worth about 6,000 US$ while a 2.00 ct diamond, of similar quality, may cost 15,000 US$. Large diamonds are much more rare and valuable than smaller diamonds. A 1.00 ct diamond weighs the same as four 0.25 diamonds. Even if all four value factors are equal, a large diamond is much more valuable than the small ones together.

Differences in colour and clarity can make small diamonds more valuable than large diamonds. The difference in size of those diamonds is more significant. A 0.5, colour grade D, Flawless diamond is more valuable than a 1.01 ct, colour grade P, I3 diamond.