A carat (ct.) is the unit of measurement specifically used to describe the weight of a diamond (or other gemstones). Its name comes from the carob seed – a small seed with a typically uniform weight that early gem traders used as counterweights for balancing their scales, according to the GIA.

A single carat is equivalent to 0.2 grams, or 200 milligrams, and is divided into 100 points.AA certified diamond that comes with a grading report will indicate the exact carat weight to the nearest hundredth of a carat, in decimal format. A 1-carat diamond has 100 points (1.00) while a ¾ carat stone has 75 points (0.75). Carat weight in pre-set jewelry is typically described as a fraction (e.g. ¾3 carats) and has an equivalent decimal range (¾3 carats = 0.69 – 0.82 points). The following table correlates fractional sizes with their decimal equivalent.

The carat is probably the most familiar of the 4C terms because it is the easiest one to understand just by looking at the stone. However, people often mistakenly assume that a diamond’s size is synonymous with its weight, though that’s not necessarily true. The way a diamond is cut can actually obscure its size and true weight.

It’s important to note that it’s not just the carat weight, but also the quality of the stone at that weight that helps determine the diamond’s value. Factors that determine quality include the cut, color, clarity, and finish. One exceptionally high-quality diamond can sell for $20,000 per carat while a lower-quality one sells for just $1000 per carat.Diamond values also increase disproportionately to the size of the stone, since larger diamonds are more rare. In other words, a three-carat stone with equal color, clarity, and cut can end up costing significantly more than three times the cost of a one-carat stone.