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

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

# Ordinary ray

Also called o-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.

## In more detail

The ordinary index ω is 2.648 in moissanite, 1.770 in ruby and sapphire, 1.583 in emerald and 1.544 in amethyst. With the table cut across the axis, the o-ray gives one of the two refractometer edges.

## How we measure it

Our Dichroscope shows the o-ray color in one window when the dichroscope lines up with the stone. In ruby the o-ray is purplish red; in emerald it is yellowish green.

## Related terms

- [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.
- [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

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

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