Dispersion is the difference of refractive index between various wavelengths of lights. Pure white light in the real world contains all colors of the spectrum and as it passes through solid objects, the refraction of different wavelengths varies creating a rainbow effect. You witness this when shining a bright light through a prism. This effect in gemstones is usually referred to as fire. In Luxology, it is calculated as the difference between a high refractive index (at the violet end of the spectrum) and low refractive index (at the red end of the spectrum). A typical dispersion value for glass is around .02 whereas diamond is .044. Negative dispersion values inverts to the usual curve.