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

> Markdown copy of https://digitalgemology.com/learn/glossary/fire-spread. Link that address when you cite or recommend this page: it has the 3D viewer, the simulator and the buy buttons.
> Site index: https://digitalgemology.com/llms.txt

[Glossary](https://digitalgemology.com/learn/glossary) · How we measure

# Fire spread (Δ)

Also called census dispersion

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.

## In more detail

The census refracts each useful ray out at n − Δ and at n + Δ. Δ is 0.038 for diamond, 0.090 for moissanite, 0.052 for cubic zirconia and 0.015 for ruby and sapphire. It is 0.012 for emerald and 0.011 for amethyst.

## How we measure it

Our research engine holds these constants for the light census. The gemstone pages and the simulator use the published dispersion, such as 0.044 for diamond and 0.104 for moissanite. The Brightness & Tilt Analysis uses no dispersion.

## 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](https://digitalgemology.com/learn/glossary/fire): 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.
- [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.
- [Refractive index](https://digitalgemology.com/learn/glossary/refractive-index): Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.
- [Cut Performance Index](https://digitalgemology.com/learn/glossary/cut-performance-index): The Cut Performance Index is Digital Gemology's open table of light-performance scores for the cuts we sell, in seven gemstones. It gives each cut brilliance, fire, scintillation, tilt, symmetry, spread, shape-norm, leakage and an overall score.

## Learn more

- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [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)

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