Carat is a measure of weight. One carat (1 ct) equals 0.20 grams and is divided into 100 points (so a 1/2 carat − or 0.50 carat − diamond can also be described as a 50 points diamond).
As diamonds increase in size, they become increasingly rare in nature. As rarity is an important component of value, a one carat diamond will be worth a great deal more than two 1/2 carat diamonds of equal color, clarity and cut.
It is important, however, to remember that a diamond is not valued by carat weight alone and two diamonds of equal carat weight can have very different values, depending on cut, clarity and color.
WHAT TO LOOK FOR:
A diamond can appear larger (or smaller) than its actual weight, depending on its cut. Your diamond may actually have a higher carat weight than a bigger-looking diamond with a shallower cut.
Look at different designs and mountings as they can affect the appearance of the size of the diamond.