The weight of a diamond is measured and counted in carats. This metric system was adopted at the start of the 20th century. Carat commercially abbreviated to ct is divided into 100 points. A carat is also subdivided into grains 1 carat equals 4 grains and a quarter carat is 1 grain.
When you purchase a diamond in a ring. The term 1-carat not only refers to a diamond of exactly 1-carat but there are weight ranges that have the same name. As you will see in the list below.
One carat (4 grainer) = 95points to 105 points
Three quarter carat (3 grainer) = 70 points to 76 points
Half carat (2 grainer) = 47 points to 56 points
Quarter carat (1 grainer) = 23 points to 26 points
Diamonds are very often cut by weight to gain the maximum weight and least loss from a rough diamond. This affects the cut and fire from the stone. A very thick girdle can add relatively a lot of weight to a diamond. Unfortunately this also effects the fire, as a lot of light will go out through the girdle. This should always be checked as it does effect the stone quite badly.