Not all gemstones have the ability to disperse light enough to be seen with the naked eye like diamonds. Diamond experts use the term “diamond fire” to describe the dispersion of a diamond's light. “Fire” here is defined as the rainbow of colors visible when moving a symmetrical diamond under the right light.
“Fire” is obtained from the arrangement of the faceted faces and the angles between them. So, although every diamond has the same dispersion value, not every diamond exhibits the same “fire.” There are four factors that interact between diamonds and light that contribute to the “fire” you see on the face of a diamond.
The first is the angle at which light hits the diamond. As soon as white light hits a diamond, the spectral colors begin to spread and separate. The greater the angle, the greater the refraction. For example, if the angle of incidence is 1 degree, the difference between the angle of refraction of purple and red is very small, less than one hundredth of a degree, and so will not reflect much “fire.” At the maximum inside angle (24.5 degrees) the spread is almost half a degree, and the “firelight” is a little more visible.
The second is the number of times a ray of light interacts with the faces in the diamond. Dispersion increases every time light is reflected or refracted in a well-cut diamond. When light rays are dispersed multiple times in a diamond, the color spectrum becomes more diverse and clearer.
The interface between diamond facets also affects “ diamond fire ”. When light rays enter a diamond, especially at a shallow angle, it begins to separate into spectral colors. If color wavelengths fall on opposite sides of a junction, the colors can separate and follow completely different paths. Each new set of spectral colors contributes to diamond fire. The more this happens, the brighter the diamond fire becomes.
The final factor is the angle of the light rays as they exit the diamond. Basically, the smaller the exit angle, the larger the refraction angle. This means the color bands will spread further, creating the appearance of larger flare. Some cuts have an increased number of top facets to take advantage of this effect.
The type of lighting can also affect fire. Diffused light, such as that produced by fluorescent lights, reflects off surrounding walls and furniture to hit the diamond from all angles. This type of light helps enhance the diamond's brilliance but does not bring diamond fire.
Directional light, also known as illuminance or point light sources, hits the diamond from only certain angles. This type of illumination contrasts with surrounding dark areas to ignite fire within a diamond. The combination of fluorescent and incandescent lighting creates a balance between brightness and fire.
You can only see “fire” if scattered light waves pass exclusively into your eyes. This does not always happen, because, just as white light scatters into colors as it enters the diamond, colored light can recombine into white light as it exits. Because diamonds are most appreciated when face up, cutters will adjust the cut so that the diamond's fire is most clearly shown on the face. That's why the best way to view diamond fire is to tilt and change its direction.
Sometimes background colors (from fabric color or contrasting colored objects surrounding a diamond) can contribute to the appearance of diamond fire.