The relation of carat and value of a diamond
The heavier a diamond, the more valuable. Still, the price of a diamond does not increase in proportion to its WEIGHT IN CARAT. Large diamonds are extremely rare. Therefor, a two-carat diamond is worth more than twice as much as a one-carat diamond.
Specific sizes also bring a jump in prices. For example, a 0.95-carat diamond costs only a little more than a 0.90-carat diamond. The price difference between a 0.95-carat and 1-carat diamond is, in comparison, significant. This applies for diamonds (and other gemstones) of 2 carat, 3 carat, and more.