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I'm Charles Beaudet, a second-generation jeweler. As I grew up my father taught me diamond grading. According to my father, before the 1920's diamonds were relatively scarce and there was no standardized grading system. Grading was limited mostly to clarity issues. If a stone was obviously tinted it was also graded lower.
Once the supply was guaranteed, the rarity of a diamond would mainly be determined by its purity of color. The finest stones were saved for viewing on a clear day from a northern facing window. Blue and yellow magically cancel each other out in the crystal. Because diamonds are mostly tinted by nitrogen and hydrogen, which adds yellow, they used the blue from the sky to on a clear day to grade their color. Those stones that the blue from the sky dominated were called Blue White. The stones where the blue was canceled out were called White. Yellow dominated stones were given names of diamond mines, such as Cape.
By the 1940's the flaw in this system was obvious. Almost 16% of diamonds fluoresce a color, most commonly blue. Meanwhile the Gemological Institute of America (GIA) was being formed as a training school for gemologists and jewelers. They developed grading tools and devised a standard system for the grading of diamond color. It used artificial daylight without the ultraviolet to avoid florescence and a yardstick technique of master stones. Today this is the recognized standard throughout the world.
Today color and clarity are graded using these standards under strict lighting, magnification and measurement. Both GIA and the American Gem Society (AGS), have concluded 10 year studies on diamond proportion, symmetry, polish and light handling, which in aggregate we call cut.
Most of the beauty of a diamond comes from its cut. By cut I'm referring to the proportions, symmetry and polish of a diamond. These are what determine its light handling ability.
When cutters are given a rough diamond, the job they have is to get the most amount of money for it that they can. They cannot affect its color or its clarity. All they can do is impact it's beauty and weight. They also determine how much time they will spend on that particular gemstone. Most of the time the final price of the diamond is determined before it is even looked at. The shape of the rough especially limits how a diamond can be cut.
The highest grade for a diamond cut is Zero. This grade is for Proportion, Symmetry and Polish (that order is important. There are 11 grades of cut (zero to 10) possible on an AGS report. Less than three and a half percent of diamonds that receive the highest grade of Zero.
The most simple way to understand the AGS grading is to use the ASET viewer which displays the overall light handling of a diamond by using a three colored cone. The light reflects off the cone onto the diamond and provides a view of the colors and symmetry. The middle color is red. The red reflected light is that which can be controlled by the cutter.
The GIA equivalent grading system uses Excellent as an overall cut grade for Polish and Symmetry. There are five possible cut grades in a GIA report.
A diamond with the top rating from either AGS or GIA that also displays a perfect heart and arrow effect (proving that the facets aim at each other in such a way that it produces the maximum fire or prismatic effects) are truly the finest diamonds cut today.
The traditional rating for most aspects of a diamond not graded by AGS or GIA are rated Excellent, Very Good to Excellent, Very Good, Good to Very Good, Good, Fair to Good, Fair and Poor. Most diamonds cut are rated as Good.
At Beaudet Jewelry We refer to "top cut" for diamonds that are even stricter than the highest laboratory grades. The quality of a diamond's cut can account for 55% of it's cost. Buying a diamond without understanding cut could be a big mistake.
Much of a diamonds cost comes from the rarity of larger sizes. There are 100 points to a one carat diamond, 50 points is half a carat. There are certain weights where prices for each point increase. A 49 pointer is less for each point than for a 50 pointer, or half carat. The difference between a 99 pointer and a one carat of 100 points can be $1000. This means a generic diamond labeled as a carat, but not a specific weight, can range from under 100 points to over 100 points. Most likely it's under 100points. It also means that the odds of finding a top cut diamond in a 99 pointer is extremely small. Cutters would seldom find that profitable.
Most of the rarity of a diamond comes from its color. The stones free of any body color are the highest grade. These are D, E and F. They are colorless and when held up to a blue sky the blue from the sky can reflect through it unaltered. G, H and I are considered White or near colorless. Lower colors J-K and L are slightly tinted and can show a yellowish or brownish tint when worn, because the added color of body oil which touches the stone can make the it look slightly dingy. Commercially, K, L and M are commonly sold diamond that may look okay when clean in normal lighting, unless compared with white or colorless diamonds. When slightly dirty these diamonds can look dingy.
Clarity makes a diamond both beautiful and rare. VVs, or very very slightly included, can take an expert using familiar equipment up to 20 minutes to locate under 10x magnification. VS can take up to 5 minutes. SI-1 up to the count of three. S-I2 is a large grade. It can encompass stones with inclusions that are only visible when the stone is dirty to stones slightly diffused in appearance by several inclusions that are deflecting light. I-1 and I-2 may have inclusions visible when looked at with the naked eye or may have a slightly shattered look.
The important cut-off is at the point where the light is not affected by the inclusions. In SI-1 and higher grades the inclusions do not affect how light travels through the stone and therefore do not affect its beauty. In an emerald cut, or a poorly cut diamond of another shape, an SI-1 might barely be seen under some lighting circumstances. In a fine cut diamond this will not happen. Generally speaking on a diamond less than 1 carat, an SI-1 inclusion that is strictly graded by GIA or AGS will not be visible and will not affect a diamond's beauty.
When buying a diamond you decide what is right for you. However remember that much of it's value is determined by what is imperceptible in a showroom. Unless you understand what you are buying you may not know what you are giving up, therefore it is always advisable to use the services of a trusted, professional jeweler.