The human eye is exquisitely sensitive to slight differences between two diamonds, but only in direct comparisons. Viewed separately, many diamonds, especially those toward the high ends of the three "C" scales of Color, Clarity and Carats, look alike to most observers. Any diamond with Color between D "Colorless" and H "Near Colorless" appears colorless, unless it is placed next to a much lighter stone. Diamonds in Clarity grades from F "Flawless" to VS-2 "Very Slightly Included" have no visible flaws unless viewed by an expert under 10x loupe magnification. A diamond weighing 0.9 carats is obviously smaller when seen right next to a 1.1-carat stone, but viewed separately, few people can distinguish them. But it is the fourth "C", its Cut, which can make a diamond stand out as a "sparkler" that even an amateur will immediately notice. Well-proportioned diamonds seem to float slightly above a supporting surface and to glow of their own power. This characteristic overcomes even rather severe shortcomings in other features such as polish or symmetry.

Even though a diamond's Cut has the most profound effect on its overall beauty, it has traditionally been regarded as impossible to determine objectively. Ostensibly to remedy this, AGS and GIA have devised extremely complex grading systems that they have started to include in their formerly objective quality reports. Despite all their promotion and hoopla, these so-called Cut Grades are not in fact especially helpful in selecting the best diamond, and are often seriously misleading.

The images above illustrate the simplest example of how the path of a single light ray entering the top table facet is redirected by internal reflections in differently crafted diamonds. If the cut proportions are either too shallow or too deep, a large portion of incoming light "leaks" from the bottom pavilion facets, reducing the amount returned to the viewer. Exactly which combination of cutting angles provides the most return of light entering at all angles has been debated since Marcel Tolkowsky's landmark analysis published in 1919. A century of opinion from the appraisers of millions of diamonds has resulted in empirical ranges of proportions agreed to give the best results. For example, the most desirable depth percent for the Round Brilliant is in the range 59-63% (see, e.g., the clear explanation at Certified-Plus Diamonds).

Each diamond shape, such as round, marquise and square, has been assigned these ranges of proportions that are most desirable in terms of “brilliance” and “fire”. Based on these values, a generally accepted set of formulas is used by appraisers to estimate the weight of a diamond from its width and height, as measured with a caliper such as the ubiquitous Leveridge Gauge. If all of the other proportions, such as the table percent and pavilion percent, are also within their most desirable ranges, the formula gives an estimated weight that is equal to the actual weight. If not, the estimate will be high or low.

Any competent laboratory report includes very reliable values for actual height, width and weight. This provides all the information needed to compare its estimated weight and actual weight. The proprietary e-Praise Cut Score (eCS) formula does this comparison by computing their ratio. If they are equal, the eCS is equal to 1.000. A lesser score, such as 0.992, indicates that the diamond's proportions give it less than maximum sparkle by being cut either too shallow or too deep, or to be otherwise out of proportion.

The e-Praisal formula applies value adjustments for each of the features of shape, finish, polish, culet size, girdle thickness and so forth that are recorded in the laboratory quality report. These relative value corrections are well established in professional appraisal practice. The geometric proportions are treated separately. By adjusting for eCS along with all the other characteristics, the monetary value results closely parallel the seemingly "subjective" opinion of expert appraisers. The concept of this approach may be simply interpreted. It is tantamount to giving the value of the largest properly cut diamond that is contained within the subject diamond.

This is not to recommend the actual re-cutting of a low-scoring diamond, but that buyers are encouraged to choose gems as close to eCS=1.000 as possible. This makes selecting the "most for your money" a simpler task.

Any prospective buyer should know a diamond's eCS. Every high-quality diamond should include a Diamond e-Praisal certificate as independent scientific evidence of its craftsmanship as well as its current monetary value.