A layman’s perception of a natural diamond is a bright, sparkly, round object cut from a rough diamond. Few, however, understand the science behind diamonds and their enduring beauty. While the best sales associates have a solid grasp of the four Cs—carat, cut, clarity, and colour—many, even among the most experienced, may not fully appreciate the complexities of light performance.
Schematic showing the difference between light return/reflection and light leakage.
Everyone knows that diamonds sparkle, but light performance measures how well they sparkle. Quite simply, light performance in a natural diamond refers to how well the stone interacts with light, affecting its visual appeal. To achieve excellent light performance scores, the path of light must be examined, including entry, refraction, internal reflection, path through diamond, exit, and visual effects.
Refraction refers to the bending of light when it enters the diamond—light slows down and bends due to the diamond’s high refractive index (approximately 2.42). This bending of light allows for the striking visual effects that diamonds are known for. Reflection, on the other hand, refers to how the light reflects or bounces off of the diamond’s facets. The diamond’s proportions, angles, and overall cut significantly influence how much light is reflected to the observer. Well- placed facets can maximize light return, enhancing the diamond’s brilliance. There is also internal reflection. Diamonds are designed to take advantage of total internal reflection. When light hits the facets at certain angles, it reflects inside the diamond instead of escaping, thereby contributing to the diamond’s brightness.
Modern technology, such as the American Gem Society’s (AGS’s) Angular Spectrum Evaluation Tool (ASET) and Sarine Light device, can now measure how well a diamond interacts with light by analyzing the entire path of light. Sarine’s 21st-century light performance technology measures a combination of brightness, sparkle, fire, and light symmetry. AGS’s ASET technology similarly measures brilliance, fire, scintillation, and contrast.
Brilliance is a word we all use with diamonds, but the scientific definition of brilliance is the amount of white light reflected back to the viewer. A diamond that reflects more light back to the viewer will have a higher brilliance score and a better light performance (see Figure 1). The score for fire (see Figure 2) measures light that is dispersed into colourful flashes back up towards the viewer. The scintillation (or sparkle) score measures the sparkle of the diamond, which is created by the spots of light flashes when the diamond is in motion. There are two types of scintillation: (i) flash scintillation, white sparkles that dance across the diamond’s facets, and (ii) fire scintillation, colourful, rainbow sparkles that appear in the diamond’s crown. The important factor of contrast also needs to be considered. Contrast is defined as a diamond’s alternating bright and dark areas, similar to a hall of mirrors. All of these factors together create the final grade for light performance. Diamonds with lower light performance scores generally have light leakage, meaning light is refracted off to the side or through the back of the diamond (see the far left and right diamonds in Figure 1) rather than being reflected back to the viewer. This is why some diamonds seem to sparkle more than others and/or have more brilliance.
The ultimate light return or “light performance” from a natural diamond stems from a combination of two important factors. The first is the type of rough diamond it was cut from and its transparency. The second is the method of cutting the rough to create the final polished diamonds.
Natural diamonds mined from the earth’s core are formed in numerous types of shapes. Some popular rough diamond shapes include octahedrons
Diamond light performance report
The second and most important factor affecting the light performance of a polished natural diamond is its cut. How a diamond is cut—its unique arrangement of facets—greatly affects the amount of light reflected back to the viewer. GIA studied diamond cuts for decades and analyzed tens of thousands of proportion sets before their cut grading system was introduced in 2005. It had to be scientific, practical, and applicable to the jewellery industry and the public. There were more than 70,000 observations of 2,300 diamonds in studies conducted across all sectors of the jewellery industry—diamond manufacturers, dealers, retailers, and potential customers. The research behind their cut grades is undeniable. The quality and method of cut will greatly impact a diamond’s light performance score. The combined effects of proportion, symmetry, and polish can greatly influence how well a diamond reflects and refracts light. Well-cut diamonds maximize light return and minimize light leakage, leading to greater overall beauty (see Figure 1). The brightest diamonds with optimum light performance receive a cut grade of ‘Ultimate’ by Sarine, achieving quadruple 0’s in brilliance, sparkle, fire, and symmetry (see Figure 2). AGS’s ASET technology is another renowned way of measuring light performance in a diamond (See Figures 3, 4, and 5. Many jewellers are not as familiar with these so-called “green-grey maps” as they could be. They show a scan of the diamond and how light behaves within the diamond. Red is for brightness. Whenever you see a lot of red, it means the diamond is extremely bright. Blue is for contrast. Contrast is necessary for the diamonds to sparkle. Green areas on the maps show where the diamond is less bright. All diamonds will have some areas of green, but the highest light
dodecahedrons, cubes, and triangles. At the heart of a diamond’s brilliance is its crystal lattice structure. This structure is made of tightly packed carbon atoms. This arrangement is key to the incredible properties of the diamond, such as the diamond’s hardness and how it handles or refracts light. As most savvy sales professionals know, diamonds are the hardest natural substance, with a rating of 10 on the Mohs Hardness Scale. The transparency of rough diamonds is more influenced by their internal structure and inclusions than by their external shape. However, octahedral diamonds are typically more common and often exhibit fewer inclusions than dodecahedral rough diamonds
Understanding the rough is the first step to creating higher light performance. Cutting facets to maximize the transparency is key. When cut well, octahedral rough diamonds with very few inclusions typically exhibit better light performance because of their internal structure and natural transparency.
performance scores will go to the diamonds with less green. Grey on these diamond maps shows light leakage. Light leakage in a diamond, as explained above, is not desirable. The goal in cutting the diamonds is to minimize the light dispersal from the sides or bottom and instead have most of the light reflect from the top of the diamond back to the viewer. The AGS ASET’s standards of cut are based on a 0-10 scale, whereas GIA uses a more generalized scale of 5 (excellent, very good, good, fair, and poor). Let’s consider Figures 3, 4, and 5. Figure 3 represents a diamond with more light leakage around the edges (more grey). Figure 4 represents a diamond with lots of green, meaning it is less bright. Figure 5 shows a Fireworks Diamond® with little to no light leakage (almost no grey) and exceptional brightness (lots of red). These three diamonds may have similar ratings of the four Cs on their GIA reports, but clearly, the light performance in Figure 5 is by far the most
A diamond exhibiting increased light leakage around the edges, resulting in more grey areas.
A diamond with significant green areas, indicating lower brightness.
optimal. The ‘0000’ score signifies zero deduction (or light leakage) out of the diamond by AGS cut standards.
Being able to speak professionally and somewhat scientifically about light performance positions you as an expert and will earn the customer’s trust. Your expert guidance is the one thing that online retailers cannot offer. When you show customers diamonds with higher light performance scores alongside diamonds with lower light performance scores (and similar four Cs), your customer can see the difference. You can talk about the higher-quality rough from which the brighter diamond started. You can then explain the precision of the cut and how master diamond cutters create more sparkle or higher light performance scores and minimize light leakage. All of this builds confidence in the customer and can help you achieve higher margins and better selling prices.
Jay Mehta is the vice-president of Varsha Diamonds, a 35+ year diamond and jewellery manufacturer in Los Angeles. A second-generation diamantaire, Mehta’s expertise spans the full diamond cutting process, from rough diamond assorting to perfecting cuts. He founded Fireworks Diamonds®, the only diamonds scientifically proven by AGS’s ASET and Sarine Light performance technologies to be the largest and brightest in the world. Fireworks is one of the few brands awarded a proprietary cut grade by the American Gem Society Laboratories.
A Fireworks Diamond with minimal light leakage (almost no grey) and exceptional brightness, characterized by an abundance of red areas.