---
title: "Uniaxial: meaning in gemology"
description: "A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial."
url: https://digitalgemology.com/learn/glossary/uniaxial
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Uniaxial

A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.

## In more detail

In a uniaxial stone, the ordinary ray has one index in every direction. The extraordinary ray's index changes with direction. Moissanite and emerald belong to the hexagonal crystal system; ruby, sapphire and amethyst to the trigonal.

## How we measure it

The Polariscope's conoscope shows a uniaxial interference figure, and its red plate reads the optic sign.

## Related terms

- [Biaxial](https://digitalgemology.com/learn/glossary/biaxial): A biaxial stone is a doubly refractive stone with two optic axes. It can show three colors in a dichroscope, and its interference figure differs from a uniaxial one.
- [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.
- [Optic sign](https://digitalgemology.com/learn/glossary/optic-sign): The optic sign of a uniaxial stone tells which of its two rays is slower. It is positive when the extraordinary index is higher than the ordinary one, and negative when it is lower.
- [Interference figure](https://digitalgemology.com/learn/glossary/interference-figure): An interference figure is the pattern of colors and dark bands that a doubly refractive stone shows through a conoscope. Each point of the figure is one direction through the stone.
- [Crystal system](https://digitalgemology.com/learn/glossary/crystal-system): A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.

## Learn more

- [Refractometer](https://digitalgemology.com/about/how-we-measure/refractometer)
- [Polariscope](https://digitalgemology.com/about/how-we-measure/polariscope)

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