Describing diamond beauty -
assessing the optical performance of a diamond
Abstract:A diamond's "optical performance" is a human visual interaction with the diamond reflecting and refracting surrounding light to its viewer in a sparkling mosaic pattern. It is this "light performance" of the diamond that results in its beauty. This work advances the idea that the best diamond cuts "evolved" to have the highest optical performance in a variety of typical viewing and illumination circumstances.
Current and historical descriptors of diamond beauty and performance are reviewed. The rationale is put forward for the need to augment the current, single, quantitative measure of brilliance - light return intensity or "brightness" with the qualitative aspect of "contrast brilliance." Contrast brilliance is explained, and illustrated with diamond photography. The relationship between contrast brilliance and scintillation is examined.
When a gemologist or appraiser evaluates a diamond, it is rated in four categories: cut, colour, clarity, and carat weight. Known as the "4 C's", these are the characteristics that are assessed when valuing a diamond based upon its beauty and rarity.
Of the four C's there is general agreement that cut has the greatest influence on the diamond's beauty. The term cut may bring to mind any of three features of a polished diamond.
- Shape such as round, rectangular, pear, oval, or cushion.
- Style of facet pattern such as radiant, step cut or princess cut.
- The angles and proportions of a particular style and shape.
This article confines the meaning of cut to the angles and proportions of a particular shape and style of a diamond, such as the familiar, 57-facet round brilliant. There are several elements to the quality of a diamond's cut.
Judgments of the elements of cut fall in two distinct categories.
The first category is craftsmanship. Examples of measures of craftsmanship are assessments of perfection of polish, facet meet points, facet alignment, symmetry, facet angles and proportions. Most grading of diamond cut being done today makes judgments that fall within this category. There is the implicit assumption that a diamond with superior craftsmanship will have superior optical performance or beauty. From their knowledge and experience, diamond cutters know that a high level of craftsmanship in cutting and polishing a diamond to specific angles and proportions results in superior performance and beauty.
The second category, more recently introduced, is a "direct assessment" of the performance aspects of diamond beauty. Direct performance assessment contrasts with the indirect assessment of cut quality in the first category, because it is an actual rather than implied measure of aspects of diamond beauty. The main aspects of diamond beauty assessed are brilliance, fire and sparkle. This category of direct performance assessment evaluates the extent to which all the various factors of craftsmanship have or have not actually resulted in a beautiful diamond. Direct performance assessment augments and validates the traditional category of craftsmanship.
Diamond beauty - brilliance, fire and sparkle
The quantity together with the "quality" of the light returned from a diamond to the eye of the viewer is critical to the viewer's judgment of the diamond's performance and beauty.
A well-cut diamond has the ability to reflect colours and light from a broad range of surrounding directions and angles. A well-cut round brilliant reflects and refracts the surrounding panorama of light to the viewer's eyes in a mosaic, even artistic, composition possessing vibrancy and vitality.
This article refers to the visual interaction of the diamond reflecting and refracting the surrounding light to the viewer as the diamond's "optical performance" or "light performance". This optical performance results in the diamond's beauty. The beauty that results from this unique performance has stirred emotions and engendered devotion unsurpassed by any other gemstone.
Historical and contemporary terms describing diamond beauty
The historical words, brilliance and fire, and the more recent word, scintillation, are three terms that have evolved to describe diamond beauty. These are the words widely used in the diamond industry by gemologists, diamond cutters and sellers to convey aspects of diamond beauty with simplicity and clarity.
The most beautiful diamonds have "the most vivid fire and the greatest brilliancy" proclaimed Marcel Tolkowsky, the historically influential Belgian diamond cutter and mathematician. Both he and his contemporaries used these two terms, brilliance and fire, to describe the beauty of a diamond.
The third descriptor of diamond beauty called scintillation has since been added. Combined with "contrast brilliance", a related fourth aspect of diamond beauty introduced by this article, the jeweler is armed with the necessary vocabulary to convey the beauty of one diamond cut compared to another.
Brilliance
The Diamond Dictionary (Gaal, 1977) defined brilliance as: "the intensity of the internal and external reflections of white light to the eye from a diamond or other gem in the face-up position."
It is important to note here that brilliance was defined as the "intensity of light return". This "brightness" measure, which is a single value, lacks any detail of important variations in intensity across the diamond. This intensity variation or "contrast" is needed to complete the picture of the historical meaning of brilliance.
Fire
Called the diamond's "fire", an important aspect of the quality of light return is the ability of a diamond to disperse white light into the rainbow colors of its spectrum.
Scintillation or Sparkle
Characterization of diamond beauty by fire and brilliance leaves all features other than fire as aspects of brilliance. A definition of brilliance narrowed to the single quantitative measure of average brightness necessitates additional descriptors of other quality aspects of diamond brilliance.
Scintillation is one such descriptor. Scintillation is the word used to describe a quality aspect of brilliance historically referred to as the diamond's "life". It is the diamond's sparkle occasioned by movement of the diamond, the illumination or the observer. This quality of light return is the sharp, on-off, bright-dark sparkle or flashes of light "dancing" from the crown of the diamond.
The three terms used today to describe diamond beauty - brilliance, scintillation and fire were the respective qualities in this quote by Tolkowsky from Diamond Design, 1919, p.24. "The general trend of European diamond polishing is the constant search for greater brilliancy, more life, a more vivid fire in the diamond, regardless of the loss of weight". These are the qualities that diamond cutters of Tolkowsky's day were seeking in their quest for the most beautiful diamond cut. Maximizing these qualities of diamond beauty has remained the goal of diamond cutters from that time to the present.
Contrast quality of brilliance
In addition to scintillation, another descriptor is needed to augment the single, quantitative definition of brilliance as average brightness.
Human perception of brilliance goes beyond the brightness of the light returned from the crown of a diamond. Intensity variation or contrast in light and dark areas across the diamond gives it an aspect of brilliance that has been described as "snappy", "lively" and by Bruton (1978, p.227) as "hard" or "sharp". These descriptions are the opposite of "watery" and "glassy" used in The Diamond Dictionary (Gaal, 1977) to describe the weak appearance of a poorly cut, "fish eye" diamond.
The term "contrast brilliance" is adopted to describe this aspect of brilliance. Contrast brilliance is the intensity variation or contrast in light and dark areas across the diamond.
The term is needed because this aspect of diamond beauty influences our judgment of which diamond cuts are the most brilliant. Recognizing this contrast aspect of brilliance gives us a means for explaining why one diamond cut may be perceived as less brilliant than another even though it may have had equal or greater average brightness.
It is worth noting that an emerald cut diamond is normally perceived to be less brilliant than the round brilliant cut mainly because it has poorer contrast brilliance even though it may have similar brightness.
A second need for the introduction of this aspect of brilliance that will become apparent involves its relationship to scintillation. Scintillation will be shown to be the dynamic form of contrast brilliance. Because of its dynamic nature, scintillation eludes measurement, but contrast can be measured from static diamond images. The relationship between contrast and scintillation enables an assessment of both, through the characteristics they have in common.
Simultaneous contrast
The contrast aspect of brilliance is linked to the property of human vision called "simultaneous contrast".
When a bright reflection is close to a darker one, our vision enhances the perception of the difference, and the bright reflection looks brighter because of its proximity to the darker reflection. The greater the contrast, the brighter the diamond appears to our eye.
One consequence of the phenomenon of simultaneous contrast is our attention to variation or contrast in a scene. For example, we are generally attracted to greater contrast in a picture. Note the example in Figures 1 and 2. Most people would agree that the greater contrast in Figure 2 makes the photograph more appealing.
 Figure 1. Photograph of a scene printed with less contrast. |
 Figure 2. Photograph of Figure 1 printed with greater contrast. |
The following experiment is designed to illustrate and support the idea that contrast in a diamond influences the perception and judgment of brilliance. The diamond image in Figure 3 was printed with maximum contrast. The same diamond image in Figure 4 was printed with less contrast between the bright reflections and the adjacent dark reflections.
Using the current quantitative definition of "brightness", the Figure 4 image would emerge as having the greater brilliance. However, we would probably all perceive the Figure 3 diamond image as more brilliant. The perception of greater brilliance in Figure 3 is due to the white reflections in that diamond appearing brighter and more intense because of greater contrast with the dark reflections. Actually, the white reflections in both Figures 3 and 4 have the same intensity.
 Figure 3. Diamond image printed with maximum contrast but a lower light-return intensity. |
 Figure 4. Diamond image printed with less contrast but a greater intensity of light return. |
This demonstration illustrates and supports the influence and importance of contrast to the perception and judgment of brilliance.
Dependence of diamond beauty upon viewing and illumination circumstances
Both the brightness and contrast aspects of brilliance depend not only upon the cut of the diamond, but also upon its illumination and viewing circumstances.
 Figure 5. Diamond exhibiting high average "brightness" but low "contrast brilliance". |
 Figure 6. Same diamond exhibiting high contrast brilliance. |
Figures 5 and 6 are two photographs of the same diamond illuminated in different ways. In Figure 5 the diamond was uniformly illuminated from above with no obstruction from the presence of a viewer. This resulted in a diamond image with high light return brightness but low contrast. This type of uniform illumination, lacking contrast, results in an unfamiliar and undesirable diamond appearance, which also lacks contrast.
The illumination of the diamond in Figure 6 consisted of high contrast lighting. As a result this well cut diamond exhibited a high contrast aspect of brilliance.
Figures 5 and 6 have demonstrated that both the brightness and contrast aspects of brilliance are interrelated with the diamond's illumination. It is often overlooked that the perception of diamond beauty in all its aspects is greatly influenced by the character of the illumination.
Evolution of Excellence in Diamond Cutting
This brings us to the important point that the perception of superior beauty in a diamond derives from the ability of its cut to take greatest advantage of the lighting under which it is being viewed and judged.
The development of diamond cutting judged most beautiful evolved through "cut and try" experimentation. Because success was judged under standard or typical illumination, (discussed below), which was not uniform, the finest cutting inevitably evolved to have the greatest beauty in those typical viewing and non-uniform illumination circumstances.
This article advances the idea that diamond cuts judged to have superior optical performance and beauty possess brighter, more numerous and evenly distributed, both large and small, higher contrast reflections in that variety of typical illumination circumstances.
The worth of a measure of diamond beauty depends upon how well it agrees with human judgment. --- "Perception of beauty is everything." --- To be meaningful, measurements of diamond beauty should be made in illumination typical of circumstances in which human judgment of that beauty is made.
Measurements of beauty in atypical lighting can give high scores to cuts that have lower scores in typical illumination. For example, measurements of brilliance in uniform lighting (such as that illuminating the diamond in Figure 5) can give high brilliance scores to cuts that have lower perceived brilliance in typical illumination.
Typical Illumination and Viewer Obstruction
There is a short list of key characteristics of typical viewing conditions that have influenced the evolution of the most beautiful and highest performing diamond cuts. A key, ever-present feature in typical illumination circumstances is the influence of the presence of the viewer on the diamond's illumination. Although this "viewer obstruction" at first sounds disadvantageous, in well-cut diamonds it most often adds contrast to the illumination that is favorable to beauty. That same viewer obstruction results in poor optical performance in a badly cut diamond.
For example, in Figures 7, 9 and 10, where there is open sky illumination, it is viewer obstruction that introduces contrast to otherwise diffuse illumination. This illumination is seen mirrored in the ring's prong in Figure 8. The prong acts as a convex mirror reflecting the entire panorama of illumination. The prong mirrors the overcast sky,which is partially obscured by the silhouette of the viewer's head, torso and outstretched hand and arm. As the ring's prong reveals, it is obstruction by the viewer that introduces contrast to the otherwise diffuse illumination.
 Figure 7. Face-up photograph of a 2.25ct diamond exhibiting brilliance and blue fire occasioned by bright, overcast sky partially obscured by the viewer's silhouette. |
 Figure 8. The illumination of the diamond in Figure 7 as seen mirrored in the ring's prong. Notice that the overcast sky is partially obscured by the silhouette of the viewer's head, torso and outstretched hand and arm. |
The dark appearance of the viewer in the prong is the factor providing the primary contrast in illumination. It is the synergy of the diamond's cut taking advantage of this contrast in illumination that produces both the contrast and fire present in the diamonds in Figures 7,9 and 10. Without contrast in its illumination the best diamond cut will lack both contrast brilliance and fire, as Figure 5 demonstrates. In typical viewing circumstances that always include the viewer's presence, the superior beauty of the best-cut diamonds becomes apparent.
The factors that characterize the contrast quality of brilliance are the sharpness, number, sizes, and uniformity of the distribution of the diamond's mosaic-appearing pattern of reflections.
How can we assess this new quality of contrast brilliance?
The answer that derives from the previous discussion is straight forward. All these aspects may be observed and evaluated from stationary images or "snapshots" of a diamond under a representation of typical viewing and illumination circumstances. Because diamonds are most often viewed in the face-up position, the most important snapshot to examine for contrast brilliance is this normal, face-up view (See Figure 10). Snapshots at other angles of observation representing usual tilts of the diamond also need to be examined.
 Figure 9. Tilted round brilliant cut diamond displaying fire and contrast brilliance under the partially obscured illumination of bright, overcast, sky. |
 Figure 10. Face-up view of a round brilliant cut diamond displaying fire and contrast brilliance under the partially obscured illumination of bright, overcast, sky. |
Relationship between the contrast quality of brilliance and scintillation
What is the difference between contrast brilliance and scintillation? Clearly, the diamond's mosaic pattern of reflections has aspects common to both contrast and scintillation (i.e. aspects such as sharpness, number, sizes, and uniformity of distribution of the reflection pattern). However, the contrast quality of brilliance is the diamond's "static contrast", whereas scintillation is the "dynamic contrast" due to movement. Contrast brilliance is one frame or snapshot of the moving picture of scintillation. In fact, the change in contrast brilliance with movement from one moment to the next IS scintillation. Contrast brilliance and scintillation are perceptually and mathematically related in this way. (Mathematicians may identify scintillation as the partial derivative of contrast brilliance with respect to movement)
Conclusions
These examples, illustrations and photographs support the need for the additional descriptor of contrast brilliance. Contrast brilliance complements the brightness or intensity aspect of brilliance, which is the current widely used definition.
Recognizing the contrast aspect of brilliance gives us a means for explaining why one diamond cut may be perceived as more brilliant even though it had less overall brightness or intensity of light return.
The established language for describing diamond beauty is in terms of brilliance, fire and scintillation or sparkle. Contrast brilliance adds another dimension to the measure of brilliance, which is currently confined to the average brightness or intensity of light return. Evaluation of contrast in snapshots of diamonds gives us a "handle" on the more difficult to measure, dynamic aspect of scintillation.
Recognition and judgment of this additional quality aspect of brilliance helps complete the picture of why one diamond cut is preferred over and is seen as more beautiful than another.
References
Bruton E. (1978) "Diamonds", Chilton Book Co., PA, 532 pp.
Gaal R. (1977) "The Diamond Dictionary", Gemological Institute of America, California, 342 pp.
Tolkowsky M. (1919) "Diamond Design", Spon & Chamberlain, New York, 104 pp.
