Carat is the unit of measurement used to describe the weight of loose diamonds. One carat weighs 200 gms and just as a dollar is divided into 100 pennies, a carat is divided into 100 points. Therefore a stone weighing 1.50 carats has a weight of 1 carat and 50 points.
Diamonds of the same weight do not necessarily have the same appearance. When diamonds are mined, large gems are discovered much less frequently than small ones, hence making them much more valuable.
Diamond prices rise exponentially with carat weight. So a 2 carat diamond of F colour, VVS1 clarity and excellent cutting will always be worth more than two 1 carat diamonds of the same colour, clarity and cutting.
Also, a 0.50 carat diamond with high color and clarity ratings may cost more than a 0.75 carat diamond with lower color and clarity ratings. As diamond carat size increases, both the diameter and the depth of the diamond increase. This is why a 1 carat diamond (approx 6.5mm diameter) does not look twice as wide as a 0.50 carat diamond (approx 5mm diameter).