The standard unit of weight for diamonds is the metric carat, which equals 1/5 gram. That’s approximately 7/1000 ounce. For even greater precision, the carat is subdivided into 100 equal units called points. In the United States, diamond weight is measured to 1/1000 and rounded to the nearest 1/100 carat. This means that the weight is accurate within ½ point or 35 millionths of an ounce. Both industry standards and the Federal Trade Commission require this extreme measurement precision.
Carat weight is usually stated in decimal numbers like 0.50 and 1.25 carat or in common fractions such as half, quarter, three-quarter, etc. A retailer of diamonds may describe the fractions by saying “point four eight carats” or “forty-eight hundredths” of a carat or “forty-eight points,” or round up by saying “about half a carat.” (All of these are different ways of saying the same thing about a stone.)
A carat’s effect on value depends on natural rarity. A 1-carat diamond is much rarer than two 1/2-carat diamonds of similar quality. Therefore, the larger diamond may be more expensive than the two smaller diamonds combined.
Other differences in rarity are reflected by a diamond’s per-carat price, which is the cost for each carat. This can be helpful in comparing two or more stones. One diamond may cost $3,000 per carat while another costs $5,000 per carat due to the other qualities of the stone such as color, clarity, and cut.