The weight of a diamond is measured in carat. One carat (1ct) corresponds to 0,20 grams.
The weight in carat is one of the most important points to define the price of a diamond, but it is not the only one. Two diamonds of the same weight will still have no same value, because the price will also be value of the other elements which establish 4C (clarity, cut or color).
It is also necessary to know that the value of a diamond is not proportional in its weight, the more a diamond will be big, the more its price by carat will be important, because the diamonds of several carats are very rare.