A Carat is the unit of measurement for a diamond's weight - it is equal to 200 milligrams, and there are 142 Carats to an ounce. Carats are referenced by dividing them into points - there are 100 points in a Carat.
A half Carat diamond may be referred to as a 50-point stone (about 100 milligrams). Because large diamonds are rarer, they generally have a greater value per Carat than smaller sized stones.
This word for the measurement of a diamond's weight, Carat, is derived from the carob seeds that were used to balance scales in ancient times.
Today, it is important to evaluate all of the factors (all the C's) and not just size. A large sized diamond with poor clarity, color, and cut does not have the value of many smaller stones. C's like clarity, cut, and color determine the visual brilliance of jewelry.