From promise rings to tennis bracelets, the one thing people know most about diamonds is that carat weight and size are loosely equated. Basically, the larger the diamond the more it weighs.
The weight of a diamond is expressed in carats. One carat is fixed at 0.20 grams. One carat is divided into 100 points. A diamond measuring one quarter of a carat, can also be described as weighing 25 points, or 0.25 carats.
The rarity of a diamond is greatly affected by its size. The rarity of a 1.00 carat diamond is much greater than twice that of a 0.50 carat diamond. Although it only weighs twice as much, the 1.00 carat diamond is statistically much more rare and difficult to mine than the 0.50 carat.
Here are some examples of different weights for round diamonds and their corresponding sizes.