All transparent materials, in addition to refracting light, also exhibit what is called dispersion, that is, the refractive index is slightly more for the bluer component of daylight than for the redder. The different colours which constitute white light are differently refracted, i.e. dispersed, and this is of course why a glass prism shows the colours of the rainbow. In diamond, the dispersion is especially high, about five times that of glass. So what is the result when light falls on the well cut diamond First, 18 per cent is immediately reflected. Then the rest enters, but is largely reflected within the diamond and ultimately finds its way back to the eye of the beholder. But on its path it is strongly dispersed, i.e. split into brilliant, widely separated spectral colours. This, then, constitutes the famous fire of the diamond, the flash of spectral colour from the dispersed light spectrum.

It need only be added that fire is far more effective in flickering candlelight than in daylight or electric light. The changes in angle of incidence of light resulting from the flickering candles make a diamond jewel worn in such light appear to be virtually alive and flashing with fire. Truly the thousand candles at Versailles, before the days of gas or elecricity, must have brought out to perfection the brilliant fire of the diamonds worn by the ladies of the French Court.

—S. Tolansky, from The History and Use of Diamond, 1962.