---
title: "Extraordinary ray: meaning in gemology"
description: "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,…"
url: https://digitalgemology.com/learn/glossary/extraordinary-ray
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

> Markdown copy of https://digitalgemology.com/learn/glossary/extraordinary-ray. 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) · Optics

# Extraordinary ray

Also called e-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.

## In more detail

The extraordinary index ε is 2.691 in moissanite, 1.762 in ruby and sapphire, 1.577 in emerald and 1.553 in amethyst. Along the optic axis, the e-ray has the same index as the o-ray, so there is no split.

## How we measure it

The simulator traces the e-ray with 1/n(θ)² = cos²θ/nₒ² + sin²θ/nₑ². Our Dichroscope shows its color: orangy red in ruby, bluish green in emerald.

## Related terms

- [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.
- [Birefringence](https://digitalgemology.com/learn/glossary/birefringence): 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.
- [Optic axis](https://digitalgemology.com/learn/glossary/optic-axis): An optic axis is a direction in a doubly refractive crystal along which light does not split into two rays. Seen along it, the stone behaves as if it were singly refractive.
- [Pleochroism](https://digitalgemology.com/learn/glossary/pleochroism): Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.

## Learn more

- [3D Gemstone Simulator](https://digitalgemology.com/about/how-we-measure/simulator)
- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [Dichroscope](https://digitalgemology.com/about/how-we-measure/dichroscope)

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