Fire

Fire is the dispersed colored light you see when a diamond or gemstone catches a directional light source — sunlight through a window, a candle, a single overhead spot. White light entering the stone is refracted at different angles by wavelength, splitting into the visible spectrum: red, orange, yellow, green, blue, violet. The result is flashes of pure color that seem to originate from within the stone.

The phenomenon is identical to what a glass prism does. Diamond is simply a very good prism — its high refractive index and specific dispersion value (0.044) create intense spectral separation. That number, the dispersion value, is a fixed property of the material. It cannot be altered by cutting. What cutting controls is how much of that potential fire reaches your eye — and under what conditions.

Dispersion is fixed by material. But the cut controls two things that determine how much fire you see in practice: the crown angle and the crown height. A steeper crown angle increases the range of angles at which light enters and exits, amplifying spectral separation. Lower crown heights reduce it. This is why crown angle sits at the exact intersection of brilliance and fire — adjusting it shifts the balance between white light return and colored spectral output.

The practical tension: brilliance and fire compete for the same light. A stone optimized purely for fire (steep crown, smaller table) will appear slightly less bright. A stone optimized for brilliance (flatter crown, larger table) will show less fire. The round brilliant, as Tolkowsky calculated in 1919, tries to optimize both simultaneously. Most buyers get it right without realizing: the ideal round brilliant crown angle of 34–35° is specifically the zone where brilliance and fire are balanced, not where either is maximized.