CARAT (ct.): is the unit of measurement used to define the weight of a diamond. The word is taken from the carob seeds that people used in ancient times to balance scales. Do not confuse the terms Carat and Karat.
CARAT: A measurement of weight. One metric carat is equivalent to 0.2 grams (or 200 milligrams).
KARAT: A measurement of gold purity.
POINT: A carat equals 1/5 of a gram (or 1/142 of an ounce); each carat is further divided into points, each point representing 1/100th of a carat.
In the final analysis, weight may be the least important of the Four Cs in determining value. It's sort of like buying a painting based solely on how much it weighs. The Mona Lisa is quite light, but it's going to cost you a lot more than a heavy mural painted by your neighbor (even if your neighbor is a great artist).
Carat weight is often thought of as an indication of a diamond's size, however as you've learned here, it is in fact a weight measurement. The final size of your diamond is determined by the combination of carat weight and cut. It's a good idea to consider all of the Four Cs when determining the value of your diamond, not just how much it weighs.
Here is a map to show you the approximate size of a round diamond at each of the following carat weights:
Although diamond weights can be estimated when they are mounted in jewelry, the exact weight may only be obtained when the diamond is loose. In fact, gemological laboratories such as the Gemological Institute of America's (GIA) Gem Trade Laboratory and American Gem Society Laboratories (AGS) will only grade loose, unmounted diamonds.