Four criteria are used to evaluate the quality of colorless diamonds in the most objective manner possible: the 4 C's. These are Carat (weight), Clarity (purity), Color, and Cut. In order to classify diamonds in a clear and uniform way, laboratories and other international certificating bodies have agreed a common set of precise parameters to be used in assessing each of these criteria. This classification is very important as it influences the sales price of diamonds.
1. Carat (weight)
The value of a stone increases exponentially with its weight. The weight of a diamond is measured in carats: 1 carat is equal to 0.2 grams, and is divided into 100 points. The word "carat" is derived, via Arabic and Italian, from the Greek word "keration" (fruit of the carob). Because of their uniform weight, carob seeds were in the past used on precision scales, and different countries each had their own carat. In 1907 the metric carat of 200 milligrams was adopted, and is universally used today.
The "carat" used for gems is not the same as the "karat" used for gold. The latter refers to the ratio of pure gold contained within a metal. This ratio is calculated in twenty-fourths, i.e. pure gold will be 24 karats (24/24 = 100%) whereas 18 karat gold contains 18/24 of pure gold (i.e. 75%).
Here are several ways to express 1 carat :
- 1 ct.
- 200 milligrams
- 1/5 gram
- 100 points
- 4 grainer
2. Clarity (purity)
This is a very important step in determining the value of a stone. Specifying purity involves making an inventory of all the internal and external impurities of the stone and evaluating the one that is the most representative and the most important. The diamond is examined under a microscope (30x) and with a magnifying glass (10x). Purity grades are (in decreasing order):
IF (internally flawless) : completely pure diamonds
VVS1 - VVS2 (Very Very Small inclusions) : tiny inclusions can barely be seen through a ten-power magnifying glass, even by a trained diamond grader.
VS1 - VS2 (Very Small inclusions) : very small crystals, clouds, cracks or pinpoints characterize this grade. These inclusions are not very visible and have no impact on the sparkle of the diamond.
SI1 - SI2 (Small Inclusions) : impurities can be seen with a 10x magnifying glass but are not visible to the naked eye.
I1 - I2 - I3 (GIA terminology) or P1 - P2 - P3 (HRD terminology) (Piqué) : impurities are visible to the naked eye.
Colorless diamonds are assessed against a range of colors that goes from colorless (D - for "Diamond") to light yellow or brown (P - the most yellowish color). The color is observed through a uniform, Northern daylight 65 light, and is compared to a set of reference master stones. "+" or "-" signs are added in order to fine-tune the definition of the color. A K- stone for example will have a color similar to but slightly less yellow than a K stone.
D - J :
colorless or white diamonds.
K - L :
"tinted white" diamonds.
M - P :
"tinted color" diamonds.
Almost 100 years ago, the Master cutter Marcel Tolkowski defined the ideal proportions that govern the most popular cut: the brilliant (or round). Indeed, the ideal brilliant has to be conceived and cut in such a way that the light that enters the diamond through the sides of the crown is amplified by the pavilion (which acts as a perfect mirror) and exits the crown through the table. In order to obtain this effect, a number of "ideal" parameters were defined which do leave some margin for maneuver: the height of the stone in relation to its diameter, the depth of the pavilion, the angle of the pavilion, the angle of the crown, the height of the crown...
After having studied these various parameters, a grade is given to the cut of the stone as a whole: VG (Very Good), G (Good), M (Medium) or P (Poor).