Diamonds are admired for their sparkle. A common question is whether they shine in the dark. Diamonds do not produce their own light and therefore do not “shine” in darkness. They rely on external light sources to display their visual characteristics.
The Brilliance of Diamonds in Light
Diamonds derive their appearance from how they interact with light. Several optical properties create their brilliance and “fire.” These properties require an external light source to be visible, transforming light into a dazzling display.
Refraction is the bending of light as it passes from one medium to another. Diamonds have a very high refractive index (2.417 to 2.419), causing light to slow down and bend significantly upon entering the stone. This redirects light efficiently within the diamond’s internal structure.
Dispersion, or “fire,” is another optical phenomenon. When white light enters a diamond, its high dispersion (0.044) causes it to split into spectral colors, much like a prism. This creates flashes of rainbow colors.
Reflection also plays a role, categorized into brilliance and scintillation. Brilliance is the white light reflected internally and externally from the diamond. Scintillation describes the flashes of light and contrasting dark areas visible as the diamond, light source, or observer moves. These combined optical effects give diamonds their sparkle, but they depend on ambient light.
Diamonds and Darkness
In darkness, diamonds do not glow or emit light. Their sparkle depends entirely on an external light source. However, some diamonds exhibit fluorescence, which can be mistaken for self-luminosity.
Fluorescence occurs when certain diamonds absorb ultraviolet (UV) light and re-emit it as visible light. This glow is most commonly blue, though it can also be yellow, green, white, or orange in rare cases. This effect is not continuous; it requires a UV light source, such as a blacklight or sunlight, to activate, and ceases immediately once the UV source is removed. Not all diamonds fluoresce; approximately 25% to 35% of natural diamonds display some degree of fluorescence.
A rarer phenomenon is phosphorescence. While fluorescence stops as soon as the UV light source is removed, phosphorescence allows a diamond to continue glowing for a short period afterward. This “afterglow” typically lasts from a few seconds to several minutes. Like fluorescence, phosphorescence is a reaction to an external UV light source. Without any light, visible or ultraviolet, a diamond will appear as a dark, non-luminous object.