---
title: "Corundum: meaning in gemology"
description: "Corundum is the mineral species made only of aluminum and oxygen. Its red variety is ruby, and all its other colors are sapphire."
url: https://digitalgemology.com/learn/glossary/corundum
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Corundum

Also called aluminum oxide

Corundum is the mineral species made only of aluminum and oxygen. Its red variety is ruby, and all its other colors are sapphire.

## In more detail

All corundum shares one set of properties: refractive index 1.762 to 1.770, birefringence 0.008, hardness 9 and specific gravity 4.00. Its crystal system is trigonal. Corundum forms only where silicon is scarce, and silicon is common, so natural corundum is relatively uncommon. It has excellent toughness and no cleavage.

## On a grading report

LMHC wording prints '(natural) corundum' as the identification, and 'ruby' or 'sapphire' as the species or variety.

## Related terms

- [Ruby](https://digitalgemology.com/learn/glossary/ruby): Ruby is the red variety of the mineral corundum. Chromium causes its red color.
- [Sapphire](https://digitalgemology.com/learn/glossary/sapphire): Sapphire is corundum of any color other than red. The word alone means a blue stone; other colors take a color word, such as yellow sapphire.
- [Gem species](https://digitalgemology.com/learn/glossary/gem-species): A gem species is a mineral with one chemical composition and one crystal structure, such as corundum, beryl or quartz.
- [Gem variety](https://digitalgemology.com/learn/glossary/gem-variety): A variety is a subdivision of a gem species, usually by color. Ruby and sapphire are varieties of corundum, and emerald is a variety of beryl.
- [Hardness (Mohs)](https://digitalgemology.com/learn/glossary/hardness): Hardness is a gem's resistance to scratching. Gemologists rank it on the Mohs scale, on which diamond is 10.
- [Refractive index](https://digitalgemology.com/learn/glossary/refractive-index): Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.

## Learn more

- [Ruby 3D models and cutting files](https://digitalgemology.com/gemstones/ruby)
- [Sapphire 3D models and cutting files](https://digitalgemology.com/gemstones/sapphire)
- [Dichroscope](https://digitalgemology.com/about/how-we-measure/dichroscope)
- [Refractometer](https://digitalgemology.com/about/how-we-measure/refractometer)

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