Diamond Brilliance

Introduction

When we think about what makes a diamond beautiful and special, two words usually come to mind; Fire and Brilliance. Although the terms are often used interchangeably, and although the two aspects are related, they are actually two distinct optical properties.
As a basis for understanding the distinction you can think of diamond sparkle as consisting of a combination of white flashes and colored flashes. Colored flashes are the result of light dispersion – light that is broken into spectral hues by refraction. This is commonly referred to as “fire”. To learn more about this magical property please see our companion article on diamond fire. Brilliance is the term generally used to describe white light return, which is the foundation of a diamond’s brightness and sparkle.
But to be brilliant a diamond must be more than just bright. Think of a big mirror reflecting light back to your eye. It is blindingly bright, but not pleasing. Contrast and scintillation are necessary for a diamond to produce the right combination of optical effects to be brilliant and to be pleasing to the eye.
For a diamond to be brilliant a number of factors are required. It must be cut and polished in a way that light can be gathered and redirected to the eye efficiently. If the diamond is not cut to the proper proportions, light will leak out through the pavilion and brightness will be lost. The cut design also must feature enough structured contrast to result in the on/off blinking of the facets in motion. This sparkling effect is referred to as scintillation and is required for the diamond to appear brilliant.
The diamond must be crafted in a way to exhibit contrast, such that some facets are dark when others are lit. This creates the on/off blinking of the facets when the diamond, the observer or the light source is in motion. The contrast must be balanced and well distributed. Too much contrast and the diamond will look dark. Too little contrast and the diamond will lack scintillation.
AGS Laboratories has developed the leading cut grading system in use in the diamond industry. It is a scientifically peer reviewed system that analyzes the light performance of the diamond based on a computerized evaluation of the contribution of all its facets. In all, 11 aspects of cut quality are evaluated in the AGSL grading system, including the light performance aspects of brightness, contrast, dispersion, and light leakage. The system assigns a grade from 0 to 10, with 0 being Ideal. A transparent diamond with Ideal brightness and Ideal contrast will have outstanding brilliance.

Contents

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Brilliance and Scintillation

The evaluation of scintillation is rather complex and involves calculating the size, frequency, and duration of sparkles through a range of tilt angles. There are variables involved in the cut design and execution that can alter the size of the reflections to create bigger bolder flashes or smaller ones, and also alter their frequency as the diamond is viewed in motion. To an extent, these variations in pattern can be considered ‘taste’ factors. Some may prefer a more subtle twinkle, while others respond better to bold flash. Largely for this reason, AGSL does not specifically grade scintillation on their standard light performance reports. However, the grading of the other light performance factors indirectly takes scintillation into account. Diamonds that get the AGS Triple Ideal grade have an optimized balance of brightness, contrast, and fire, with minimal light leakage. Their facets are also symmetrical and polished to a mirror finish.
While scintillation patterns might be considered a taste factor, research suggests that the human brain is wired to respond more positively to certain contrast patterns and frequencies. Contrast helps us distinguish important things in our visual field from the general background, a critical evolutionary adaptation.
Spatial Patterns
Most observers, when asked which portion of the image above most immediately draws their attention, will indicate an area in the lower middle section. There is an intrinsic preference for spatial patterns in the intermediate frequencies with good contrast.

Virtual Facets

An important concept in the understanding of diamond light performance is the ‘virtual facet’. A round diamond has only 57 or 58 facets (depending on whether it has a culet). But a well cut round will appear to have many hundreds of reflections. Because entering light beams can be divided (partitioned), and a physical facet can reflect light beams from multiple directions, a diamond produces a multiplicity of virtual facets. The size of these perceived facets and their number depends on the number of physical facets in the design, and on the number of times light is partitioned as it propagates in the stone. It is the number, size, and arrangement of virtual facets that determine the optics of the diamond.

Structured Contrast and Obscuration

One of the interesting insights derived from modern research on diamond light performance is the role that the observer plays in the equation - in particular, the concept of ‘obscuration’. Some light is blocked by an observer’s body, particularly the head, as the diamond is being viewed at close range. This head shadow, which is reflected back to the eye by some of the facets, results in structured contrast. If present in the right amount and distribution, this can have a positive effect on scintillation and our perception of brilliance. Obscuration is measured and mapped in the AGS light performance system and is represented by the blue area in the ASET hemisphere.
Super Ideal Cut Round Diamond

Contrast and Perception

As mentioned above, structured contrast is vital in creating the dynamic on/off blinking or scintillation. It also has another important effect on our perception. Contrast is where brightness and brilliance start to diverge. Studies have shown that humans are ‘hard wired’ to detect edges. Contrast provides us with edges, enhancing our perception of diamond brilliance. Brilliance can therefore be best described as brightness with positive contrast effects.
Super Ideal Cut Round Diamond
Bressan Chart
As you know, when seen against a white background, the gray squares should be darker. Then why are the gray squares seen against the black background darker? Can you see the complex way in which our visual system determines lightness? When the gray squares are seen as members of a group of small white squares, they seem darker than when seen as members of a group of small black squares (even though they're seen against a black background). So, it's not necessarily the immediate background that's crucial, but instead the way in which our visual system has organized the stimuli.

Binocular vision

Further impacting the perception of brilliance is the fact that we look at diamonds with two eyes that are separated in space from one another. With ‘stereo’ vision, we see different virtual facets (or parts of virtual facets) simultaneously. These separate signals themselves result in variations in brightness. These are processed by the brain where phenomena such as binocular rivalry takes place, influencing our perception.
Binocular Vision

Relative Brightness

Another interesting aspect of brilliance is that it can also be enhanced by changes in relative brightness. We know that well cut rounds that return ample amounts of high angle light (red in ASET) will appear brilliant. But fancy shapes that return more low angle light (less bright light as represented in ASET green) can also appear brilliant. The contrast between facets reflecting low angle and high angle light, even though it is not on/off contrast, can enhance our perception of brilliance. Because of the way our brain works we adjust instantaneously to the relative brightness of reflections. It is even possible that small areas of leakage that are well distributed can also create positive contrast.
So our perception of brilliance is affected by:
  • Brightness
  • Contrast caused by obscuration
  • Contrast caused by changes in brightness

Facet Design and Cut Precision

Because the virtual facet sizes and distribution are crucial to scintillation, brilliance, and fire, the design of the cut is the first critical factor. The design must have a good mix of large and small facets, and a proper amount of structured contrast, in order for light performance to be optimized.
But the full benefit of good design will only be realized by faithful execution of the design by the cutter. Precision cutting is critical to achieving the mix of virtual facets prescribed by the design. Optical precision (sometimes call optical symmetry) is the accurate alignment of all corresponding facets in 3 dimensional space. Efficient transmission of light as it propagates within the diamond and eventually back to the eye, depends on accurate alignment of the system of tiny mirrors that are the physical facets of the diamond. Finally, every facet must be polished to a mirror finish in order for light to reflect and refract throughout the diamond without degradation.
For a discussion of the various rewards that accrue to owners of this level of diamond craftsmanship, please see our article on the benefits of hearts and arrows diamonds.

Transparency and Brilliance

Clarity features, and to some extent color, can affect a diamond’s transparency and interfere with light return and refraction. Light scattering inclusions and defects in the crystal lattice can have a negative impact on brilliance and fire. Because grades at the top of the diamond clarity scale are so incremental, inclusions in the top 4 or 5 grades tend to have little or no impact. But some types of clarity characteristics in the Si and lower ranges can have an appreciable impact on transparency and brilliance.
The same is true of color to a lesser extent. When we see color in a diamond it is because certain wavelengths are being absorbed and not returned to the eye. But in the colorless and near colorless ranges, so little light energy is absorbed that the impact on transparency is negligible.

Conclusion

Pure crystallized carbon – Diamond – has tremendous potential for brilliance and fire. While clarity and color are inherent properties of nature, the quality of the cut, which is entirely determined by man, determines how much of that natural potential will be realized. Proper proportioning, symmetry and optical precision are all critical in maximizing the beauty of a faceted diamond. The design and execution of the cut will determine the net result of the complex interplay between the facets, the light source, and observer. Diamonds crafted to exacting tolerances require more time and skill to produce. They also require more of the original rough to be polished away, reducing yield and increasing cost. But the most precious gem on earth deserves an approach that makes no compromise in pursuit of all the beauty that can be brought out. Outstanding quality does not happen by accident. It is at the narrow intersection of intent, design, and craftsmanship where the magic of diamond light performance fully materializes.
Venn Diagram

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