The weight of a diamond is measured in carats (1 carat = 0.2 grams). Carat weight is one of the most important points in defining the price of a diamond, but it is not the only one. Two diamonds of the same weight will not always have the same value, because the price also depends on the other elements that make up the 4C.
Also worth knowing is that the value of a diamond is proportional to its weight, the larger a diamond will be, the more its price per carat will be important, as diamonds of several carats are very rare.