---
title: "Crystal system: meaning in gemology"
description: "A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic,…"
url: https://digitalgemology.com/learn/glossary/crystal-system
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# 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.

## In more detail

Diamond and cubic zirconia are cubic, so they are singly refractive. Beryl and moissanite are hexagonal; corundum and quartz are trigonal. These stones are uniaxial, so they can show pleochroism and blink in a polariscope. Some references count trigonal as part of the hexagonal system.

## How we measure it

Our gemstone pages give each stone's crystal system and show a unit cell of its structure.

## Related terms

- [Singly refractive](https://digitalgemology.com/learn/glossary/singly-refractive): A singly refractive stone has one refractive index in every direction, so light stays one ray inside it. Diamond and cubic zirconia are singly refractive.
- [Uniaxial](https://digitalgemology.com/learn/glossary/uniaxial): A uniaxial stone is a doubly refractive stone with one optic axis. Moissanite, emerald, ruby, sapphire and amethyst are uniaxial.
- [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.
- [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.
- [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.
- [Cleavage](https://digitalgemology.com/learn/glossary/cleavage): Cleavage is a mineral's tendency to split along flat planes in its crystal structure. A hard knock in a cleavage direction can split a stone.

## Learn more

- [Diamond 3D models and cutting files](https://digitalgemology.com/gemstones/diamond)
- [Moissanite 3D models and cutting files](https://digitalgemology.com/gemstones/moissanite)
- [Ruby 3D models and cutting files](https://digitalgemology.com/gemstones/ruby)
- [Emerald 3D models and cutting files](https://digitalgemology.com/gemstones/emerald)
- [Amethyst 3D models and cutting files](https://digitalgemology.com/gemstones/amethyst)
- [Diamond vs moissanite: two look-alikes, two different materials](https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-vs-moissanite-differences)

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