The carat is the unit of weight of a gemstone and is in fact exactly 200 milligrams. One carat is divided into 100 "points," so that a diamond of 50 points weighs 0.50 carats (or 1/2 carat). Although size is the obvious factor in determining the value of a diamond, two diamonds of equal weight can have very different values because of the difference in cut, clarity and colour.
Two different gemstones may have the same dimensions but different weights. This is due to the specific gravity or density of the gem mineral. This difference can help identify a gemstone.
The world's most precious diamond is believed to be the Koh-I-Noor diamond, set into Queens Elizabeth II's crown and weighs 105 carats.