The most important diamond characteristics that are graded and measured are diamond color, clarity, cut, and carat weight. These four factors, also called “the four Cs” have the most impact on the price of diamonds.
Apart from the 4 Cs, there are other characteristics that can be included in a diamond certificate, such as symmetry, polish, light return, fluorescence, etc.
How Is Diamond Color Graded?
Different diamond grading labs use different color grading scales, and one of the most popular is that of the GIA (Gemological Institute of America).
In this system, color grades are denoted by letters, starting from D (the highest color grade), with the lowest grade being Z.
On the GIA scale, stones graded with letters from D to F are referred to as Colorless.
The next group contains the Near Colorless diamonds, with grades from G to J.
Stones graded in the K-L-M range are called Faint Yellow.
Grades from N to R are assigned to the Very Light Yellow stones, and diamonds graded from S to Z are referred to as Light Yellow.
The color of a diamond is usually graded by comparing it with a set of benchmark stones (master stones), each of which corresponds to a certain color grade.
For example, if a stone has more color than the master diamond graded F but less color than the one graded H, then the diamond will most likely be classified as color grade G.
How Is Diamond Clarity Graded?
Diamond clarity is graded by inspecting the stone under magnification as well as with the naked eye and determining how clear the diamond is of visible flaws, or inclusions.
Inclusions can be internal or surface flaws, and their visibility depends on several factors: number, size, position, and color.
So, a stone can have many inclusions, but if they are visible only with a microscope, the diamond’s clarity grade will be relatively high.
On the other hand, a diamond can have few inclusions, but if they are big and visible with the naked eye, the stone’s clarity will be relatively low.
Also, centrally located inclusions impact clarity more, as they are more visible than flaws towards the sides of the stone.
The GIA grades diamond clarity with the following grades: IF (Internally Flawless), FL (Flawless), VVS (Very Very Slightly Included), VS (Very Slightly Included), SI (Slightly Included), and I (Included).
All these grades, except IF and FL, are further divided into finer grades by adding numbers to them, so in practice the scale becomes: IF, FL, VVS1, VVS2, VS1, VS2, SI1, SI2, I1, I2, and I3.
How Is Diamond Cut Graded?
Cut is graded by comparing a diamond’s proportions with a set of benchmarks.
These benchmarks are optimal diamond proportions that have been found to enhance some of the stone’s important characteristics such as brilliance and sparkle.
For example, some of the key ratios used to grade the cut of round diamonds are the depth of the stone and the diameter of its table expressed as percentages of the diameter of the girdle (the diamond’s widest edge).
The GIA evaluates round cuts on a scale that includes the following grades: Excellent, Very Good, Good, Fair, and Poor.
Other grading labs have scales with slightly different names, but the logic behind these systems is the same.
In general, you can expect brilliant cuts (such as the round cut) that are more highly graded to have greater brilliance, whereas lower-graded cuts will look less bright.
How Is Diamond Carat Calculated?
Carat is a unit of weight; one carat is equal to 0.2 grams, or 200 milligrams.
To find out the carat weight of a diamond, you can weigh it in grams and then convert the value to carats by dividing by 0.2. For example, a 0.5-gram diamond has a carat weight of 2.5 carats.
To convert the carat weight of a diamond to grams, simply multiply by 0.2. Thus, a 1.5-carat diamond weighs 0.3 grams, or 300 milligrams.
Diamond carat weight is an important price factor – since bigger diamonds are rarer, they are exponentially more expensive per carat than smaller ones.