Fire

Fire is the flashes of spectral colour — visible rainbow flashes of red, orange, yellow, green, blue, and violet — a faceted diamond throws when white light enters the stone and disperses into its component wavelengths. The trade also calls this dispersion. It is one of the three optical effects a cut diamond produces (alongside brilliance and scintillation) and is the effect most responsible for the perception of "life" or "movement" in a stone.

Fire is physical — diamond has one of the highest dispersion values of any colourless gemstone material (0.044 measured at standard wavelengths), which is why it shows pronounced spectral colour where most other clear materials show only white reflection. The dispersion happens at every air-diamond boundary the light crosses: entry through the crown, reflection off the pavilion mains, return through the crown to the eye. The amount of visible fire is a function of how many crossings happen and at what angles.

Steeper crown angles increase fire. The crown angle (33.5° to 35.5° on GIA Excellent rounds, 34.0° to 35.0° on AGS Ideal-0) controls the angle at which light enters and exits through the bezel facets. Steeper crowns refract more steeply and disperse more visibly. Old European cuts with 38° to 42° crown angles show pronounced fire patterns modern brilliants do not because the steeper crown geometry disperses more aggressively.

Smaller tables also increase fire. A 54% table leaves more crown area for the bezel facets to occupy, increasing the proportion of light that enters through the higher-dispersion bezel paths. A 60% table compresses the bezels into a narrow band and reduces fire correspondingly. The trade-off is brilliance and spread — smaller tables and steeper crowns trade overall light return and face-up size for spectral colour.

Fancy shapes produce different fire patterns. Step cuts (emerald, Asscher) show fewer but larger fire flashes — the "hall of mirrors" effect of long parallel facets. Modified brilliants (oval, marquise, pear, cushion modified) show fire patterns intermediate between rounds and step cuts. Old Mine and Old European cuts show the most pronounced fire of any historical brilliant pattern.

AGS Light Performance grading measures fire directly via ray-trace and reports it as the dispersion metric on the AGS Ideal Report supplement, graded 0 to 10. AGS Ideal-0 requires a 0 (top) on dispersion. GIA does not publish a separate fire metric.