There are several basic criteria according to which a diamond is graded, although interestingly there is no particular grading system which has been recognized by the International Standards Organization (ISO). However, there is an overwhelmingly accepted system, and it is the one developed by the Gemological Institute of America (GIA).
The GIA was established in 1931 by Robert M. Shipley to advance and teach the fledgling discipline of gemology in the United States. It was there that the system known as the four Cs was developed and codified. It involved a diamond grading and evaluation program based on the carat weight, clarity, color and cut of diamonds.
The four Cs form the basis of any diamond grading report, or, as they commonly are called in the trade, diamond grading certificates. Once issued mainly for more expensive goods, today they are commonly obtained for a wide variety of merchandise, and since the advent of online sales have become essential tools to accurately report and verify vital information about diamonds that cannot be examined physically.
The price of a diamond is determined by the relative values of its four Cs. In other words, a diamond 1.00 carat, with a good color, a high clarity grade and a great cut, will cost considerably more — sometimes in excess of 50 percent — than a diamond of the very same weight, but with a lower color, a lower clarity grade and a poor cut.
The four Cs, which form the basis of any grading report, are as follows:
Caratage is the easiest of the 4 Cs to understand and measure, inasmuch as it refers to weight. Furthermore, because the density of diamonds is essentially constant, as the caratage of a diamond increases, so does its relative size.
Clarity refers to the purity of the diamonds, or to the degree to which it is free of blemishes and imperfections. In principal, as the clarity of the diamond increases, its value is enhanced. The clarity of a polished diamond is affected by both external blemishes and internal imperfections or inclusions, some of which were created in nature when the diamond was formed and others during the cutting and polishing process. Internal inclusions include piqués, or dark spots, gas bubbles or lines, and even tiny crystals.
The following are the commonly used gemological terms to define clarity:
The majority of natural diamonds fall within a color range that runs from colorless (sometimes called white) to near- colorless and then to lightly yellowish or brownish. Slight variations that occur form the basis of a generally accepted color grading system that uses the letters of the alphabet, in which D is the highest grade, and it is assigned to a completely colorless or white diamond. Near-colorless diamonds are graded with the letters E and F. The most common color grades encountered by diamond consumers run from the color grade G through the color grade M. Diamonds that display a slight yellowish or brownish hue receive the color grades K, L or M. The color grades N, O, P, Q and R represent stones with a progressively light yellowish tint, while the grades S down to Z represent diamond that show an increasingly yellowish or brownish hue.
When diamonds are of a truly vivid color, outside the D to Z scale, , they are defined as fancy colored diamonds. Here, the alphabetical color scale does not apply and they generally are described according to their hue, and the intensity of the color.
The quality of the cut, decides how the light entering the polished diamond through its table – the largest area, facing upward – will react, and consequently how brilliant the stone will appear. In the GIA’s cut-grading system, the lab calculates the proportions of those facets that influence the diamond’s table-up appearance. These include brightness, which refers to the internal and external white light reflected from the stone; fire, which is the scattering of white light into all the colors of the rainbow; and scintillation, which quite simply is the amount of sparkle a diamond produces, and refers to the pattern of light and dark areas caused by reflections within the gemstone.
Gem labs also take into account the design and craftsmanship, and the quality of polish on the facets. The GIA Diamond Cut Scale contains five grades ranging from Excellent to Poor.
Gem labs also test other aspects of the diamond, including fluorescence, which refers to a tendency of certain to glow when exposed to ultraviolet light, which can make stones in the mid to lower color ranges appear up to one grade whiter. The labs will also test whether the diamonds have been subject to a variety of treatments, generally to improve color or clarity, and whether the stone is natural or synthetic. In the case that a synthetic stone is involved, the diamond today is likely to be graded, but the certificate will clearly note that it is not a natural stone.
GIA is the most prominent of a large number of diamond grading laboratories. While their systems are similar, they are sometimes subtle differences in terminology, as well as benchmarks. Among the more prominent grading labs are the International Gemological Institute (IGI), which is headquartered in Antwerp but operates laboratories and offices in 19 different centers; HRD Antwerp, one of most experienced laboratories in business, which operates by the rules of the International Diamond Council (IDC); American Gem Society (AGS), which pioneered the Ideal Cut Grade, and the European Gemological Laboratories (EGL).
MID House of Diamonds uses the services of most leading diamond grading laboratories, including GIA, IGI, HRD Antwerp, AGS and EGL, providing verifiable grading reports with the diamonds it sells. Clients searching MID’s massive online database are able to download and examine the relevant report for a stone under consideration.