Understanding diamond carat weight

Diamond grading is a precise science that involves several criteria. The Gemological Institute of American and the American Gem Society both grade the quality of diamonds based on what are known as the four Cs. These categories include cut, color, clarity and carat weight. Here is a guide to understand how weight effects the value of diamonds:

What is a carat?

A metric carat is equivalent to 200 milligrams, or 0.2 grams. Each full carat is divisible into 100 "points," allowing buyers, cutters and graders to refer to weight less than 200 milligrams by points. For example, a diamond that weighs 0.25 carats can be called a 25 pointer. Diamonds that weigh more than a carat are expressed in decimals and carats. A 1.05-carat diamond is the same as "one point oh five carats."

Why the name?

Gemstone traders in the ancient world used carob seeds to weigh the gems they wanted to purchase. The beans come from a tree native to the Mediterranean and were observed to be of a remarkably uniform size and weight. This is why traders decided to use the carob seeds as a scale. When a person was selling a precious stone, he placed it on one side of the scale. Carob seeds were placed on the other. The more seeds required to balance the scale, the more the gem was worth. As time wore on, the name stuck. There are roughly 142 carob seeds, or carats, to one ounce.

Why is it important?

The weight of a diamond is its most objective feature. There's no real way to argue against the carats of a diamond. Diamond clarity, cut and color are all much more difficult to detect. The GIA and AGS have detailed methods of deducing the value of the other Cs. The only issue with carat weight comes more from cut. A perfectly cut diamond that has outstanding light performance will appear to be larger than it is because of the reflections it causes. However, when placed on a scale, the carat weight cannot be argued.

How does weight compare?

Though carat weight is important in determining the value of a diamond, it is not the only measurement. If two diamonds are exactly alike in color, cut and clarity, then the carat size will determine the final cost of the gemstone. However, that usually isn't the case. The other three facets of the four Cs require a little more scrutiny to evaluate.