Let’s use the round diamond or the 57 facet “brilliant cut” as an example. The brilliant cut is the quintessential shape in which almost all diamonds are cut today.
The 57 facets create a huge number of internal pathways along which light entering the stone can travel. The light that appears to pour from a diamond under certain lighting is the result of a pattern of bouncing light. This maximizes two key aspects of a diamond’s shine – brilliance and fire. In effect, a very expensive 57 facet kaleidoscope!
Brilliance is a measure of the white light that beams from some facets of a diamond. When lit directly from above, light entering the diamond strikes one lower facet, and then a second, at near-perfect 45-degree angles. This reflects the light back out the top of the stone, so that to the viewer it seems light is streaming from inside the diamond. If the diamond’s cut is angled too steeply the light is reflected out the side of the stone. Too shallow, and it exits out the bottom.
Light striking the diamond at an angle gets refracted, or bent, as it enters the stone. As Newton discovered in his experiments with prisms, white light is a mixture of all the colors in the rainbow – and that by refracting it these component colors can be separated out. This effect, which generates the little rainbows of color that diamonds create, is called chromatic dispersion, or in gemological terms, “fire”.