"In all things of nature there is something of the marvelous." – Aristotle
If we were to look at a natural diamond, we would deem it plain and unremarkable, no more special than a piece of quartz or glass. However, when diamonds are expertly cut following precise parameters, the true nature of their unique optical properties is revealed.
The diamond cut mustn’t be confused with the diamond shape. While a diamond’s cut involves things such as the symmetry of its facets and angle measurements, a diamond’s shape indicates its form, which can be round, oval, or cushion, among others.
What defines the beauty of a diamond is its scintillating brilliance, which is the result of how this much-prized gemstone interacts with light. A perfectly proportioned diamond will act as a hall of mirrors.
In other words, while only a portion of the light that hits the diamond is reflected, the rest passes through it, bounces around inside, and bends at an angle before exiting and being reflected back to our eyes. Moreover, the diamond also acts as a complex prism by dispersing the reflected white light into the colors of the rainbow.
THE ANATOMY OF A DIAMOND
Before discussing the evolution of diamond cuts and what makes this iconic gemstone sparkle, it is best to mention the main parts of a diamond:
- The table represents the largest top facet that allows light to both penetrate and exit the diamond.
- The crown is situated at the top above the girdle and allows the maximum amount of light to enter and exit the diamond. Its multiple facets act like tiny prisms, breaking light waves into the colors of the spectrum.
- The girdle represents the part that separates the crown from the pavilion where the diamond is set in a piece of jewelry. Girdles must have the right thickness to avoid chipping or breaking and maintain the brilliance of the diamond.
- The pavilion is the bottom part of the diamond and must be properly faceted and angled to refract light. If the angle is too deep or too shallow, the light will leak out through the pavilion, diminishing the sparkle.
- The culet is the pointed bottom end of the diamond that ensures the light doesn’t leak out but gets reflected back to the viewer.
THE OPTICAL PROPERTIES OF A DIAMOND
Once light hits the surface of the diamond, three optical effects—reflection, refraction, and dispersion—come into play, giving the gemstone its mesmerizing appearance.
Thus, a portion of the light gets reflected on the surface of the facets and bounces back, being responsible for the diamond’s shine. Then, the rest of the rays of light enter the diamond and travel through it, getting scattered and fractured as they bounce around inside. This is the refraction—the diamond has the highest refractive index (2.41) of all the optical mediums, a feature that is most noticeable when it’s well cut.
Finally, when the light waves exit the diamond, they create a rainbow effect known as dispersion. Those flashes of color we see when we tilt a diamond are usually referred to as the diamond’s fire.
When describing the diamond’s light performance, we also talk about brilliance, scintillation, and pattern:
- Brilliance is the diamond’s ability to reflect the maximum amount of light back to our eyes and depends entirely on the combination of angles and proportions the gemstone is given when cut.
- Scintillation refers to the shifting sparkles of light created when the diamond, the viewer, or the light source moves.
- The pattern represents the complex arrangement of contrasting dark and light areas created by a diamond’s internal and external reflections of light.
THE HISTORY OF DIAMOND CUTTING
The Point Cut Diamond - Image Source: Gemological Institute of America
The practice of diamond cutting officially started in the 14th century, when jewelers first became aware of the octahedral form of the rough crystal and began polishing the eight natural facets of the diamond. The point cut, as it was called, was a way to remove some of the crystal’s flaws while still maintaining a natural and untouched appearance.
In the 15th century, jewelers worked continuously to perfect the cut by cutting off a part of the octahedron to create a flat table at the top of the gem and adding a culet at its bottom point. Nevertheless, it wasn’t until the 16th century when the first tools for creating facets on a diamond appeared.
The new technique was used to create a 24 facet diamond with a flat bottom and a domed top called the rose cut. Compared to the more modern cuts developed in the 18th century—such as the old mine cut—that possessed more brilliance, the rose cut offered a transparent glow as it allowed more light to move through the gem.
Each of these early diamond cuts had an essential role in developing the old European cut, the precursor of the modern round brilliant that’s so greatly appreciated today. The old European cut was used to create diamonds with 58 facets and a smaller table, larger culet, and deeper proportions than other previous cuts.
At the beginning of the 20th century, a Belgian mathematician and diamond cutter named Marcel Tolkowsky set off to redesign the proportions of the old European cut diamond in order to create a gem having the best light return. He ultimately set the basis of the ideal proportions for light performance, which were used to develop the modern brilliant diamond cut.
THE IMPORTANCE OF THE DIAMOND CUT
The Modern Brilliant Cut - Image Source: Gemological Institute of America
Apart from its high refractive properties, a diamond’s light return is also influenced by its cut. An experienced diamond cutter will generally try to maximize the brilliance, scintillation, and fire of the gemstone by giving it symmetry, proportion, and polish with the help of sophisticated precision tools and technologies.
When deciding what proportions they are going to give to the diamond, diamond cutters consider the correlation between its carat weight and diameter. On the one hand, a diamond with a small carat weight and a large diameter is cut too shallow and will leak light through the bottom of the gem. On the other hand, a diamond with a high carat weight but a small diameter will lose light through the sides of the pavilion because its cut is too deep.
Image Source: DiamondWorld
Therefore, a well-cut diamond will present a perfect balance between its carat weight and diameter measurements and reflect all the light that enters it through the crown. That’s why ideal cuts that follow Tolkowsky’s proportions have the greatest brilliance.
Image Source: Your Diamond Teacher
When it comes to symmetry and polish, a diamond’s faceting must be completely symmetrical and without any imperfections that could impair light performance.
Polishing is one of the final steps in the diamond cutting process, where diamond dust is used to smooth the gemstone’s surface so that light waves can enter and exit without any interference.
This article is part of a series where we explore the relationship between light and color in the natural world.
Antique Jewelry University - A History of Diamond Cutting
Illinois Science Council - Maximizing the Sparkle of a Diamond
GIA - Cut: The 4th C