Most diamond grading associations use the following scale for cut grading: Ideal, Premium, Excellent, Very Good, Good, Fair, and Poor. The cut grade of diamonds is a difficult process for anyone to easily assess, and yet the cut grade is one of the most important factors in choosing the right diamond. For example, one may find a round brilliant diamond with a grading from GIA of very good. This would seem like the perfect choice, but what exactly is GIA basing the Very Good on? It is also important to be aware that a cut grade report does not assess or give a value to a diamond. The cut grade report is an expert opinion on the quality of a specific loose diamond, as measured through a number of measurements and expert examination, given by an independent gemological laboratory.

Proportions

Each type of cut, whether it be round cut, princess cut or any other, has its own proportions particular to that cut. This is due to the fact that each cut has its own particular light qualities ideal to that particular cut. For example, the step cuts, such as the emerald cut, has long, plane-like surfaces. These types of broad rectangular facets would, quite obviously, not be the ideal, or even the choice of cuts in order to create the brilliant scintillation associated with the brilliant cuts, such as the ideal cut round. However, for those seeking a warmer fire and glowing light display, as well as a diamond with a longer, and often times larger looking appearance, the emerald cut, which also can enhance short fingers, can be the ideal choice. Therefore, it is not a single cut grade that is used when evaluating a diamonds grade of cut. The grade of a diamond's cut is dependent not simply on the effect of the cut on the light properties of the diamond, or on the adherence to the ideal cut ratios. The cut grade is dependent on the particular cut that diamond is filed under, i.e. emerald cut, princess cut, etc.

Polish and Symmetry in Evaluating Cut Grade

Another aspect of the cut grading is the polish and symmetry achieved in the diamond's cut. The cut of the diamond could be perfect in all regards, with all of the angles properly cut, the facets cut and angled properly, and the depth/table percentage perfect. However, even with all of that being correct, if the polish or symmetry of the diamond is off, the diamond's cut grade will suffer.

The polish of the diamond is essential to the cut grade in that it represents the ease of light to enter and exit the diamond. If the polish of the diamond is poor, the play of light emanating from the diamond's crystalline structure will neither receive the light it requires in order to produce its full effect, nor return the light to the eye with the color spectrum fully in tact. In short, the polish of the outer facets of the diamond is just as important to the cut grade as the diamond's proportions.

The symmetry of the diamond is just as important as the polish in formulating the final cut grade. The symmetry refers to the placement of the facets on the diamond's face. Even should all other aspects of the diamond be absolutely excellent, a poor placement of the diamond's facets, or the poor angling and sizing of the facets, can ruin a diamond's cut grading. It is extremely important to the diamond's light effect to have each and every facet evenly placed, sized according to the particular needs of the cut, and angled at the proper degrees.

As has been mentioned, the proportions for even the ideal cut round diamond can vary, depending upon which standard (American Standard, Practical Fine Cut, Scandinavian Standard) is used when the diamond is being graded. AGA, American Gemological Association, is considered to have the highest standards for diamond cut grading; yet they may rate a diamond that you find to have an excellent cut as simply "Good." Choosing the diamond within your budget that you find the most desirable, and then having your diamond's cut appraised by an independent appraiser is the best solution.

Ideal Cut (applicable primarily to round brilliant, and princess cut):

Many people hear the term "ideal cut" and immediately believe that this is the zenith of diamond cuts. The ideal cut can be the perfect choice, as it is the diamond cut that is designed for maximum brilliance in light quality. While the ideal cut is a good choice for many different occasions, it is not necessarily better than any other cut. The ideal cut is an excellent, and safe, cut to choose when looking for a diamond. It is not, however, the only cut in diamond to consider when looking into diamond cut. Cut is something that must be weighed along with other factors. It is also important to be aware that only the round and princess cuts are specified for the ideal cut measurements.

Below are the various "round brilliant ideal cuts" and their proportions:

The ideal cut has its origins with the mathematician and theorist, Marcel Tolkowsky, who, in 1919, published his findings in his quest for what he saw as the ideal proportions for cutting a diamond. What he used for his measurement of ideal was his measurement of light loss within a diamond. This means that when light enters a diamond from above, or the table, in order for it to provide the greatest light show, it should return back to the eye through the table. In other words, he felt that a diamond has its greatest effect when the light entering a diamond returns to the eye from the same place it entered the diamond, without any of the light leaking out of any of the other facets. It was to affect this display that he developed the mathematical equations, based on the angles of the crown, pavilion, table, depth and width, in relation to one another, that would achieve what he saw as the ideal. This ideal produced a brilliant diamond, what we now know as the category of brilliant diamonds, which include the round brilliant and the princess cut. In time, the ideal cut came to be regarded as the optimum selection in diamond cuts. This did not happen overnight, however, and many other cuts preceded the ideal cut, with many others coming after it.