The Sparkling Journey Of Light Through Diamonds

Diamonds are admired for their hardness and brilliance, with their unique optical properties causing a dazzling dance of light. Diamonds slow light to less than half its speed in a vacuum (186,000 miles per second), and refract it into a rainbow of colours. The way a diamond is cut determines its ability to transmit light and sparkle, with the facet arrangement acting as a series of mirrors reflecting the environment. This interplay of light and dark areas caused by the reflections within a diamond is what gives it its beauty and value.

Diamond's density slows light

Diamonds are admired for their hardness and brilliance. While their hardness has been known since ancient times, their unique optical properties were only recently recognized. The cut of a diamond is the most complex and technically challenging aspect of the 4Cs of diamond quality (color, clarity, cut, and carat weight) to assess. It defines the diamond's ability to transmit light and sparkle intensely, making it crucial to the diamond's final beauty and value.

The density of a diamond plays a significant role in its optical properties. Light usually travels at 186,000 miles per second, but when it passes through a diamond, its speed is reduced to less than half. This is because a diamond is extremely dense, packed with electrons, and no substance commonly encountered in everyday life has atoms more densely packed. As a result, light travels more slowly through a diamond than through any other known colorless substance.

The interaction between light and a diamond's dense structure causes light to slow down. When light travels through matter, it interacts with the electrons present in every atom, causing it to slow down and make little detours. In the case of a diamond, with its high density of electrons, light rays have to navigate through a tightly packed arrangement, resulting in a significant reduction in speed.

Additionally, the density of a diamond not only slows down light but also contributes to its ability to refract light into all the colors of the rainbow. Each time light bounces inside a diamond, it separates into its constituent colors, creating a dazzling display of brilliance, fire, and scintillation. This dispersion of light is what gives diamonds their unique sparkle and makes them stand out from other gemstones.

The density of a diamond, combined with its carefully crafted cut, maximizes its interaction with light, resulting in the captivating brilliance and fire that have come to define the beauty and value of this precious gemstone.

Light refraction and dispersion

Refraction of Light

Refraction refers to the change in direction of a light ray as it passes from one medium to another. This phenomenon occurs because light travels at different speeds in different materials. When light enters a diamond, for example, its speed changes, causing it to bend or refract. The amount of bending depends on the incident angle and the change in speed as it crosses the surface. This relationship is described by Snell's Law, which states that the refractive indices and sine of the angles of incidence and refraction are proportional.

Dispersion of Light

Dispersion is the process of splitting white light into its spectrum of colours. When white light passes through a prism, it disperses into its constituent colours, which are always in the same order: violet, indigo, blue, green, yellow, orange, and red. This dispersion occurs because different colours of light have different wavelengths, and therefore, bend by varying amounts when passing through a prism. The shortest wavelength (violet) bends the most, while the longest wavelength (red) bends the least.

The Role of Diamond Cut and Facets

The cut of a diamond refers to its facet arrangement, and it plays a significant role in how light interacts with the stone. A diamond's facets act as mirrors, reflecting light from the environment and each other. The precise facet arrangement of a diamond contributes to its brilliance, fire, and scintillation. Brilliance refers to the total light reflected from the diamond, fire is the dispersion of light into the colours of the spectrum, and scintillation is the pattern of light and dark areas and the flashes of light when the diamond is moved.

The interplay of light and facets creates a mesmerizing display of glinting lights and colours. Additionally, the diamond's appearance can change depending on factors such as lighting conditions, the observer's distance, and even the colour of the observer's clothing. This dynamic nature of light interaction with diamonds showcases the importance of understanding how light refraction and dispersion contribute to the stone's beauty and value.

Diamond's facet arrangement

The facet arrangement of a diamond is key to understanding how it interacts with light and its environment. Diamond facets are the flat, polished surfaces on a diamond that are strategically arranged to maximise the reflection and refraction of light. The precise cuts that form the shape refract light throughout the diamond, giving it a dazzling shine.

The upper portion of a diamond, known as the crown, includes the table facet, star facets, and bezel facets. The table facet is the large, flat top facet, while the star and bezel facets surround it. The lower part, known as the pavilion, consists of pavilion facets, which include the main pavilion facets and the lower girdle facets. The main pavilion facets meet at the culet, the tiny pointed facet at the bottom. The girdle, the narrow edge separating the crown and pavilion, also features girdle facets, which enhance the overall brilliance.

The ideal cut is the masterstroke that optimises a diamond’s performance with light. A well-cut diamond exhibits a harmonious balance in its facets, contributing to its brilliance. The number of facets varies based on the cut, and the arrangement is crucial for optimal light reflection. For example, the modern round brilliant cut consists of 58 facets (or 57 if the culet is excluded); 33 on the crown and 25 on the pavilion. The proportions, facet arrangement, and finish (quality of polish and quality of symmetry) constitute what is known as the cut. These factors define the diamond’s ability to transmit light and sparkle so intensely, proving the quality of the cut is crucial to a diamond’s final beauty and value.

Brilliance, fire, and scintillation

Brilliance

Diamond brilliance is the white light emitted from the stone, which is the foundation of a diamond's beauty. Brilliance has two main components: brightness and contrast. Bright diamonds perfectly return light to an observer. Diamonds with deep or shallow cuts have less brightness because the entering light leaks out of the stone. However, to be brilliant, a diamond needs more than just brightness. It should have a good amount of light contrast. For example, a sheet of white paper appears less lively than a chessboard, although it has only half of the light return of white paper.

Fire

Diamond fire, also known as dispersion, is the coloured sparkle you can see when the stone is exposed to light. It is caused by the light breaking down into spectral hues when it enters a diamond. Diamonds with small tables and steep crown angles, such as old cut stones, produce more fire because this combination also has less light return. In other words, less light return makes it easier to see colourful flashes that might otherwise be outshined by bright white sparkles (brilliance).

Scintillation

Diamond scintillation refers to the blinking flashes of light from facet to facet towards the centre of a diamond when you move the stone. An ideal diamond has many blinking flashes that spread across the surface of the stone. For example, a chessboard looks more “fiery” than a sheet of white paper when moved. Scintillation is a dynamic effect and can only be observed when there is movement between the diamond, the light source, or the observer.

Diamond's optical properties

Diamonds are a crystalline form of carbon. The addition of nitrogen atoms in the crystal structure creates defects that emit light when electronically excited. These nitrogen-vacancy centres, as they are known, can be used as single-photon sources and may have applications in quantum information processing.

The optical properties of diamonds are also influenced by their cut, which refers to the facet arrangement of the stone. The proportions, symmetry, and polish of these facets determine how a diamond interacts with light and its environment. The cut of a diamond defines its ability to transmit light and sparkle intensely.

A diamond's facet arrangement acts as a series of mirrors, reflecting the environment. A round, brilliant, colourless diamond has 58 facets that reflect its surroundings. As the diamond or its observer moves, a display of glinting lights and colours is produced as light reflects on the facets and they, in turn, reflect light onto each other. This interplay of light and dark is made up of brilliance (total light reflected), fire (dispersion of light into the spectrum of colours), and scintillation (the pattern of light and dark areas and the sparkle when the diamond is moved).

The appearance of a diamond is also influenced by its lighting environment. For example, a diamond will look different under fluorescent light compared to sunlight or candlelight. The interplay of light within the diamond is also influenced by the colour and lightness or darkness of the clothing worn by the observer.

Frequently asked questions