Diamonds are weighed on a carat scale. Trade legend has it that the weight unit's name, «Carat», is derived from the measuring unit of antiquity-the carob fruit seed, Keraton in Greek, which seeds are alleged to average 0.2 grams in modern weight standards. There are 100 points in one carat. Accordingly, a fifty point diamond is equivalent to half a carat (50/100). It is important to understand that there is no direct relationship between perceived size and weight: a one carat diamond is NOT twice as large to the eye (or as wide) as half a carat diamond. Indeed, the weight is influenced also by the diamond's total volume, which is determined not only by its diameter, but also, the depth etc.
It is also important to note that size alone is not enough to determine a diamond's value : One has to consider also the cut and proportions of the diamond, its clarity as well as its color. A large diamond holds little value if it lacks brilliance, purity and good grade color.
