Flashes of fiery light. Infinite sparkles. The hardest rock on the scale. A traditional symbol of both love and status.
Diamonds have fascinated people through the ages. Their flashy optic properties captivate the viewer and make us wonder, “What is it about a diamond that makes the light bounce around and sparkle?”
In answer to that question, we have to explore the shape, purity, and physical properties of a diamond. Let’s start with shape.
Diamonds form from carbon that has been subjected to extreme pressure and heat underground for millions of years. In their raw form, diamonds actually don’t sparkle at all. To the untrained eye, they look like any old rock. But when a trained gem cutter sees one, they begin to inspect it its weight, flaws, and optimal shape to find the best way to turn this raw rock into a collection of sparkling jewels. A jeweler usually cuts this raw diamond to maximize its weight, while cutting out any large flaws or inclusions. Large, chunky trimmings are subsequently cut into smaller diamonds.
It has been said that the round brilliant shape is the most sparkly of all the diamond cuts. The round brilliant features 57 or 58 facets, or surfaces, which allow the light to enter the gemstone and bounce around, causing flashes of light. (A diamond with a flat bottom, or culet, has 58 facets, and a diamond with a perfectly pointed culet will have 57.) To achieve this shape with maximum sparkle, the diamond must be cut at precise angles so the light entering the stone bounces between the facets and is directed back up toward the viewer.
Determining and using the optimal angles increases the aesthetic of a diamond so much that certain companies have even patented their cutting techniques. Hearts on Fire advertises that they have an algorithm that cuts every diamond at the perfect angle, and Tiffany & Co. uses a proprietary combination of angles they say is unique to their jewelry. But, what are these angles? That depends on a substance’s index of refraction, which is a measure of how much a substance bends light when it hits a surface boundary such as a facet.
And if a diamonds have optimal angles, why don’t all jewelers use the same ones?
Bending Light
Each substance has a different index of refraction, and therefore bends light at different angles. By determining the index of refraction of a diamond, jewelers have been able to determine optimal angles for shaping diamonds so that they bend the maximum amount of light back to the viewer.
Gemstones with a higher index of refraction should be cut steeper to maximize their sparkle. A higher or lower refractive index doesn’t automatically make a stone sparkle more or less. Rather, it dictates the precise angles at which the stone is cut to make light bounce in the right direction and make a stone sparkle. A stone that is cut at angles that correspond with its refractive index will achieve its own maximum sparkle. Diamond, and two well-known diamond simulants, moissanite and cubic zirconia, all have different indices of refraction;(3,4) therefore, the optimal angles for these three stones are all different, and each of these three stones should be cut differently to maximize their reflective properties. Stones with a higher refractive index, such as moissanite, should be cut with steeper angles than stones with lower a refractive index, such as diamonds (2).
Cutting Diamonds to Maximize Sparkle
The Gemological Institute of America (GIA), an organization dedicated to researching and identifying gems and the developer of the Four C’s and diamond grading scales, says that each part of a diamond’s cut contributes to its overall appearance, but there are a few particular angles that are more important than others. Specifically, a precise crown angle, in combination with a mid-range pavilion angle, and a tiny culet are sure to produce some great sparkles.
Crown Angle
The crown angle describes the angle between the flat, top surface and the crown, and the first edge that slopes downward, and is optimized between 32º and 36º (1). This angle affects the diamond’s upward appearance, or the way it flashes light back up to the viewer. While the flat octagonal table reflects white light back at the viewer, the crown facets act as prisms, and reflect rainbows of light in all directions (2). If you’ve ever sat near a sunny window and twirled a diamond ring angled at a wall, you may have noticed an octagonal light reflected from the table, as well as many tiny rainbows, which are reflected from the crown facets.
Pavilion Angle
Another angle that is critical for the diamond’s upward appearance is the pavilion angle, which describes the angle between the table and the bottom facets that meet at the culet. If the pavilion angle is too shallow, incoming light will bend too much, and it will bounce off the first pavilion angle and then reflect out of the crown, without ever reaching the other side of the pavilion. If the pavilion angle is too steep, the diamond will not bend the light sufficiently to reflect it back up at the viewer. Instead, the light will leak out of the bottom and sides of the diamond. However, at a mid-range pavilion angle (40.5º – 40.8º), light that enters the gemstone perpendicular to the table facet will bounce from one pavilion facet to the pavilion facet on the opposite side of the diamond, and then be reflected back up through the table, thus enhancing the diamond’s classical upward appearance. 
Culet
Finally, a third cut that makes a big difference in a diamond’s sparkle factor is the culet. The culet measures how pointy the bottom of the diamond is. A tiny culet helps the diamond sparkle more, and a large culet can negatively impact the diamond’s ability to reflect light upward. That said, an extremely tiny, or pointed, culet can be sharp or fragile, and needs to be set carefully in jewelry. For additional information on the other angles that contribute to the round brilliant cut, check out the GIA’s diamond guide (1, 5).
Get Involved
If you find yourself in the position to buy a diamond – or a moissanite or cubic zirconia – ask your jeweler about the angles of your stone. Some diamonds are GIA certified, and come with special reports that include information on their angles, as well as other characteristics. A reputable jeweler should know all about the physical properties of your diamond, and will likely be happy to tell you about it.
Try it at Home
If you have a diamond, hold it under a magnifying glass and see if you can spot the sides of the octagonal table facet. How many facets can you count? How pointy is the culet? When you hold the stone up in sunlight near a window, can you see small rainbows reflecting out of the crown facets?
Sources
(1) https://www.gia.edu/diamond-cut/diamond-cut-anatomy-round-brilliant
(2) https://www.prosumerdiamonds.com/crown-angle/
(3) https://en.wikipedia.org/wiki/Cubic_zirconia
(4) http://www.moissanitevsdiamondrings.com/differences-between-moissanite-and-diamond/
(5) https://www.gia.edu/diamond-quality-factor
Dana Simmons is a Ph.D. Candidate in Neurobiology at The University of Chicago, and a lifelong gem enthusiast. Follow Dana on Twitter @dhsimmons1 and dana-simmons.com.