Matters of size
When selecting a diamond, one of the “Four Cs” refers to Carat Weight. Diamonds are sold by the carat (ct), which is a unit of weight, not size.

The word “carat” is derived from the Arab word for carob, as carob seeds were used in early trading days to determine the weight of diamonds. While this method may seem unscientific, the carob seeds were so uniform in size and weight that they produced highly reliable measurements. Today, we use very sensitive, accurate scales to measure the weight of a diamond.

One carat weighs 200 milligrams, or one-fifth (.2) of a gram. This standard has been in use worldwide since 1914, when it was proposed by the International Committee on Weights and Measures. Note that the term “carat” – which is a measurement for precious gems – is different from the term “karat,” which refers to gold quality in the United States.

A few pointers
When discussing stones of less than one carat, jewelers often refer to the weight in terms of points. A carat is divided into 100 points, with one point corresponding to .01 carat. Think in terms of pennies to a dollar. There are one hundred pennies in a dollar, and there are one hundred points in a carat. So a 1/2 carat stone equates to 50 points, a 1/4 carat diamond 25 points. Very small stones, such as those used in pavé or channel settings, are sometimes called melee. Melees range from .01 to .16 carat in weight.

When isn’t a carat 100 points?
Although the analogy of pennies to the dollar suggests that one carat is always 100 points, or that one-half carat is always 50 points, that's not entirely true. Diamonds can’t all be uniformly cut to such exact weights, so the carat weight given is an approximation of the actual weight of the stone.

The impact of weight on price
Since diamonds become rarer as they increase in weight, the larger the diamond, the more valuable (and costly) it is. But the price of a diamond does not increase at the same rate as its weight. The larger the stone (all else being equal), the more disproportionate the increase in cost per carat. For example, a 2-carat diamond is always more expensive than two 1-carat diamonds of the same quality.

Magic Numbers
Small changes in the weight of a diamond can significantly impact its value. For example, a diamond weighing exactly one carat with a color grade of F and a clarity grade of VS1 could cost approximately $8,000. The diamond weighing 0.98 carats would cost $6,700. That’s a decrease in value of $1,300 for a difference of just 2/100th’s of a carat. That’s why we call weights just over 1.00, 1.50, 2.00 etc. magic numbers.