Dispersion
The splitting of white light into spectral colours because a material bends each wavelength by a different amount.
How it is measured
Gemologists measure dispersion as the difference between a stone's refractive index for red and for violet light, at two standard wavelengths. A larger number means the colours spread further apart and the stone shows more fire.
| Gem | Approximate dispersion |
|---|---|
| Quartz | 0.013 |
| Emerald | 0.014 |
| Sapphire and ruby | 0.018 |
| Diamond | 0.044 |
| Cubic zirconia | 0.058 to 0.066 |
| Moissanite | 0.104 |
Dispersion in renders
Most renderers cannot trace every wavelength of light, so they approximate dispersion by tracing red, green and blue at slightly different IOR values, or by sampling a handful of wavelengths. Either way, the result is coloured edges on light that passes through the facets.
The right value depends on the stone. Diamond should show clear but restrained colour, moissanite should show much more. Pushing dispersion far above the real value is a common way to make renders look exciting, but it misrepresents the stone and looks artificial next to a real photograph.
Seeing it
Dispersion is most visible with small, bright light sources and movement. A turntable video with a gem environment that contains a few sharp highlights shows fire far better than a soft, cloudy environment. See HDRI for how environments shape what stones reflect.