The glinting spectrum or "fire" from a colorless diamond--one of its most prized attributes as a gemstone--results from its excellent dispersion. Dispersion is the separation of white light into its component rainbow colors. The greater the dispersion, the greater the separation between the spectrum of colors that are refracted from a gem.

The refractive index can also be used to describe how visible light can be split into the colors of the spectrum when passing through a transparent substance. Essentially, this happens because the refractive index of a substance is not constant, but rather varies for different wavelengths, or colors, of light. Consequently, the shorter wavelengths of light (the blue end of the spectrum) are bent more than the longer wavelengths (the red) when entering a colorless substance at an angle. Thus, the colors separate, or disperse, producing the visible spectrum as from a prism. The coefficient of dispersion is a measure of this variation. The greater a substance's coefficient of dispersion, the greater the angular spread of colors from an incoming, inclined beam of white light--a characteristic described as a gem's "brilliance."