---
title: "Birefringence: meaning in gemology"
description: "Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays."
url: https://digitalgemology.com/learn/glossary/birefringence
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Birefringence

Also called double refraction value

Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.

## In more detail

Moissanite's birefringence is 0.043, amethyst's 0.009, ruby's and sapphire's 0.008 and emerald's 0.006. Diamond and cubic zirconia have none. On a refractometer, the largest gap between the two shadow edges gives it.

## How we measure it

The Refractometer shows two edges for stones within its range. The Polariscope uses it to compute how a stone lights up between crossed polarizers. The simulator's render traces both indices, but every score uses a single index.

## Related terms

- [Doubly refractive](https://digitalgemology.com/learn/glossary/doubly-refractive): A doubly refractive stone splits light into two rays that vibrate at right angles and travel at different speeds. Each ray has its own refractive index.
- [Ordinary ray](https://digitalgemology.com/learn/glossary/ordinary-ray): The ordinary ray (o-ray) is the ray in a uniaxial stone that vibrates across the optic axis. It has the same refractive index, ω or nₒ, in every direction.
- [Extraordinary ray](https://digitalgemology.com/learn/glossary/extraordinary-ray): In a uniaxial stone, the extraordinary ray (e-ray) vibrates in the plane of the optic axis and the light path. Its refractive index changes with direction, from ω along the axis to ε across it.
- [Refractometer](https://digitalgemology.com/learn/glossary/refractometer): A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.
- [Polariscope](https://digitalgemology.com/learn/glossary/polariscope): A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.

## Learn more

- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [Refractometer](https://digitalgemology.com/about/how-we-measure/refractometer)
- [The loupe test: doubled facet edges in moissanite](https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-doubled-facets-loupe-test)

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