At the simplest level, brilliance is reduced if light leaks out the back of a diamond. In a more complex analysis, the direction that light enters and leaves the top of a diamond becomes very important. A very deep pavilion diamond returns light straight back to the viewer, so in fact when you look face onto the table your head blocks the light and the diamond looks dull. The Holloway Cut Adviser rates diamond brilliance by taking into account the relationships between table size , and crown and pavilion angles. Fire or dispersion is the flashes of rainbow colors in a diamond.
Fire is apparent in a darker environment with a few point light sources.
You have probably seen diamonds flashing in a fine restaurant or similar environment, sometimes the source of light is just a flickering candle. But this combination also produces less brilliant diamonds. Scintillation is like black and white fire and shows up well in flood lit or office lighting environments where fire may be totally absent. The most scintillating diamonds show a star effect with eight black pointed stars radiating out from the center of the diamond. As you roll the diamond an ideal stone should show a pleasing flashes with no uneven patches.