Carat is the standard unit of weight for diamonds and other gemstone. One carat equals one fifth of gram. When weighing less than one carat, the diamond weight is expressed in "points" : each carat is divided into 100 points, for examples
0.75ct. = 75 points
½ ct. = 0.50ct. or 50 points
¼ ct. = 0.25ct. or 25 points
When diamonds are mined, large gems are discovered much less frequently than smaller ones. Diamonds prices increase exponentially with carat weight. So, a 2-carat diamond of a given quality is always worth more than two 1- carat diamonds of same quality.