Carat refers to the weight of the stone, but does not wholly encompass the actual size of the diamond. Diamond size should be measured by 1) The carat weight, 2) The proportioning of the diamond and 3) The cut grading. For example, a diamond that is one carat in weight but is proportionally smaller across the top of the stone with a poor cut grading can appear significantly smaller than an ideal cut one carat diamond. If a diamond is cut using the correct proportions a diamond of a lower carat weight, but higher cut grade can appear considerably larger than a stone with a larger carat weight with a lower cut grading.
A single carat is comprised of 100 points. So 1/2 of a carat equals 50 points. The higher the carat weight, the rarer, and generally, the more expensive the diamond will be. Diamonds can range from 1 point to 545 carats, which is the largest known cut diamond on record today called the Golden Jubilee.
The price of a diamond goes up exponentially once it reaches the full- and half-carat weight benchmarks, whereas stones with a carat weight just under the round numbers of 1 Carat, 2 Carats, 2.5 Carats and so forth, can often reflect a lower price.