Carat weight is how much the stone weighs. One carat is equal to 1/5 of a gram, or 1/142 of an ounce.

Carat weight is expressed in carats and points of a carat. There are 100 points to a carat. Therefore, 1/2 carat equals
50 points, or .50ct; 3/4 carat equals 75 points, or .75ct and so on. Diamonds are priced by weight (per carat).
That is, the weight is multiplied by the price. But the price per carat is also influenced by the other 3 C's.
The price per carat changes at pre-determined weights; which are:

.30   .38   .46   .50   .70   .90   1ct   1.5ct   2ct   3ct   4ct   etc.