Carat
A carat is equivalent to .2 grams and is the unit of weight for diamonds. The De Beers Institute of Diamonds can measure diamonds down to 1/1000 of a carat.
The value of a diamond increases in an exponential manner. Consequently, in the rare cases when two stones are of the same quality, the larger will have the greater value. A two-carat diamond for example will be worth more than two times a one-carat diamond of the same quality. Larger diamonds of high quality are extremely rare and are valued to reflect that rarity.