The 4 Cs
colour
clarity
cut
carat weight
Diamond professionals use these four factors to describe and classify diamonds. When taken together, they help in evaluating the finished diamonds you buy. That's why they are often called value factors. This page considers carat weight.
You may wish to view the excellent tutorial on the Gemological Institute of America web site titled How to Buy A Diamond
carat weight
Diamonds are weighed to a thousandth (0.001) of a carat and then rounded to the nearest hundredth, or point. Over a carat, diamond weights are usually expressed in carats and decimals. A 1.03 ct. stone, for example, would be described as "one point oh three carats," or "one oh three." A diamond that weighs 0.83 ct. is said to weigh "eighty-three points," or an "eighty-three pointer."
Some weights are considered "magic sizes" – half carat, three-quarter carat, carat. Visually, there’s little difference between a 0.99 ct. diamond and one that weighs a full carat. But the price differences between the two can be significant.
The "Per Carat" Concept
The aspect of carat weight that surprises people is the relationship between rarity, weight, and value. People know that a pound of sugar costs about twice as much as a half pound of sugar. So it’s not always easy to understand, or explain, why a 2-ct. diamond might be worth more than twice as much as a 1-ct. diamond of similar clarity, cut and colour.
It’s really a simple concept: Large diamonds are rarer than small diamonds. The scarcer a diamond is, the higher its worth. So a larger stone doesn’t just cost more. It also costs more per carat.
The larger diamond on the left is heavily included; the smaller diamond on the right is not only flawless to the unaided eye, but would be under magnification as well. Remember, as big a factor as carat weight may be, like the other three Cs, no one of them is automatically more important than the others. They all have to be factored in together in assessing the true value of a diamond.
So what is a carat?
Diamond weights are stated in metric carats, abbreviated "ct." One metric carat is just over seven thousandths (0.007) of an ounce. One ounce contains almost 142 carats. A small paper clip weighs about a carat.
The metric carat is divided into 100 points. A point is one hundredth of a carat. An easy way to remember this is to think of carats as dollars, and points as pennies. They are even written the same way: The carat first, then a decimal point, and then the points: $1.34 means one dollar and 34 cents, while 1.34-ct. means one carat and 34 points.
