All gemstones are weighed in metric carats: one carat is equal to 0.2 grams, about the same weight as a paperclip. "Carat" shouldn't be confused with "karat" which is used to evaluated the purity of gold.
Because even a fraction of a carat can make a considerable difference in cost, carat weight is evaluated very precisely. In the diamond industry, weight is often measured to the hundred thousandths of a carat, in order to be as precise as possible. There is no proportionality between the value of a diamond and its weight. A 4 carat diamond costs much more than a 2 carat diamond for example.
HOW DID THE CARAT SYSTEM START?
The carat takes its name from the carob seed. Because these small seeds had a fairly uniform weight, early gem traders used them as counterweights in their balance scales. The modern metric carat, equal to 0.2 grams, was adopted by the United States in 1913 and other countries soon after. Today, a carat weighs exactly the same in every corner of the world.