Fire is the flashes of spectral color that a cut stone sends out. In our census, fire is the share of returned light whose colors leave more than 0.6° apart.
In more detail
Fire comes from dispersion: the stone bends violet light more than red light. Our fire is a share of the returned light, not of all light. So a dim cut can still score high. Fire shows most under small, bright light sources and fades under soft, even light.
How we measure it
The census refracts each useful ray out twice, at the index minus and plus the fire spread Δ. If the two directions differ by more than 0.6°, the ray counts as fire. Fire = 100 × fire rays ÷ useful rays. The Cut Grading Tool's fire is a smooth model of crown and pavilion angle, not a trace.
Related terms
- Dispersion: Dispersion is how differently a stone bends red and violet light. Gemologists give it as the index difference between the B line (686.7 nm) and the G line (430.8 nm).
- Fire spread (Δ): Fire spread Δ is the index difference that our light census uses to split a ray into two colors. It is about 0.86 of each stone's published dispersion.
- Brilliance: Brilliance is the white light that a cut stone returns to the viewer. In our census, it is the share of entering light that leaves through the crown toward the viewer.
- Light census: The light census is our ray tracer behind the Cut Performance Wheel, the cut-page scores and the Cut Performance Index. It drops a fixed set of rays on a stone from above and counts where each one leaves.
- Cut Performance Wheel: The Cut Performance Wheel is a free Digital Gemology tool. It draws eight light-performance scores of one cut, in one gemstone, on a wheel.