A diamond weight is measured in carats, not to be confused with gold Karat (the measure of gold purity). Every carat is 100 points and 5 carats is 1 gram.
1 Cts. = 100 pt.
1 Cts. = 0.2 Gr.
Diamonds are sold per weight (carat). since large diamonds are rare to be found in nature, the larger the diamond, the more it will cost per carat.
The weight of the diamond is measured in special scales, manufactured specifically for diamonds. these scales are extremely acurate and require a very sterile environment.
The weight of the diamond can also indicate its size, however this is only an indication of the common size measurements per weight, depending on the shape and cut of the diamond. for further information on diamond size see Cut