---
title: "Fire: meaning in gemology"
description: "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."
url: https://digitalgemology.com/learn/glossary/fire
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Light performance

# Fire

Also called dispersion flashes, spectral flashes

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](https://digitalgemology.com/learn/glossary/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)](https://digitalgemology.com/learn/glossary/fire-spread): 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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/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.

## Learn more

- [Cut Performance Wheel and cut-page scores](https://digitalgemology.com/about/how-we-measure/performance-wheel)
- [Digital Gemology Cut Performance Index](https://digitalgemology.com/learn/cut-performance-index)
- [Why moissanite shows more fire than diamond](https://digitalgemology.com/learn/diamond-vs-moissanite/why-moissanite-has-more-fire)
- [Cut Grading Tool](https://digitalgemology.com/about/how-we-measure/cut-grading-tool)

[All 328 terms](https://digitalgemology.com/learn/glossary)