For a diamond to be considered "colorless," the G.I.A. requires that it be a D, E, or F. However, the D-Z scale is continuous, so the difference between an F and G is very small. The average color for engagement diamonds in the United States is G to H. Jewelers have two tools at their disposal to judge the color of a given diamond. The first is what's known as a "reference set" of stones. A jeweler will compare the stone in question with a set of stones of known color (the set is typically made of cubic zirconium!), and make a qualitative determination as to the color grading of the stone in question. The second, more precise method, is to use a colorimeter, which is nothing more than an electrical device that will measure the optical characteristics of the stone and report the color to within 1/3 of a grade. Be aware that most jewelers routinely "round up" the results of a color test. Therefore, a stone that is only slightly better than an F grade automatically becomes an E (and hence becomes more expensive for the consumer to purchase).

When judging the color of a diamond, it is crucial to see the diamond unmounted. Ask to see the printout from the colorimeter, or ask the jeweler for a reference set of stones to make the comparisons yourself. To do this, place the diamond in question next to the reference stones face down on a white piece of paper, and compare the color of the stones until you get the best match.

Cut is by far the most confusing of the 4 Cs, since it can refer to the cutting style, the shape of the stone (round, square, heart-shaped, etc.), its proportions, or the workmanship of the actual diamond-cutting process. Each of these four characteristics are important while evaluating a stone, so we will discuss each separately.

"Carat Weight" is simply the weight of the diamond. The following table shows, roughly, the appearance of different carat weights: weigh less than 1 carat, the carat is usually subdivided into "points." There are 100 points in 1 carat, so that a diamond weighing 3/4 carat would be a "75 point diamond. In summary: 1 carat = 200 milligrams = 100 points

Don't confuse the weight of a stone and its size. This argument is similar to that of cereal boxes or potato chips, where "contents are sold by weight, not volume." Also, don't confuse the carats of a diamond with the carats of a different stone. Because different minerals have different densities, a 1-carat diamond will have a different volume (a.k.a. size) than, say, a 1-carat ruby. Another common source of confusion: The karats used to measure the purity of gold have nothing to do with the carats used to measure the weight of a diamond. Notice the difference in spelling. If you're shopping for a diamond and you're unsure of a given stone's carat weight, simply ask the jeweler to weigh the stone in your presence. After making sure that the balance reports "0" when there is nothing on it, place the unmounted stone on the balance, take the number of milligrams reported by the instrument, and divide by 200 to get the carat weight of the stone.

Clarity is a measure of the number and extent of the flaws in the diamond. Generally speaking, the fewer the flaws, the more valuable the diamond. Completely flawless diamonds are extremely rare -- only a few hundred "FL" diamonds are produced per year worldwide.

There are several grading systems used to describe clarity. By far, the most popular is the Gemological Institute of America's (G.I.A.) scale, which ranks diamonds as Flawless (FL), Internally Flawless (IF), very very slightly included (VVS), very slightly included (VS), slightly imperfect (SI), and imperfect (I): between the different grades (what's the difference between "very very" slight and "very" slight anyway!):