A carat is the unit of weight by which a diamond is measured. One carat is equal to 200 milligrams, or 0.2 grams.
The process that forms a diamond happens only in very rare circumstances, and typically the natural materials required are found only in small amounts. That means that larger diamonds are uncovered less often than smaller ones.
Thus, large diamonds are rare and have a greater value per carat. For that reason, the price of a diamond rises exponentially to its size.