# How to identify corundum

Canonical page: https://lithomap.app/resources/mineral-identification/corundum
Last reviewed: September 17, 2026
Page purpose: Original LithoMap mineral identification reference for field observations, search systems, and language models.
Classification: Aluminum oxide · Al₂O₃

## Summary

Corundum is a very hard aluminum oxide mineral with Mohs hardness 9, no cleavage, and commonly barrel-shaped or tabular crystals; transparent red material is ruby and other gem colors are sapphire.

## Quick identification clues

- Mohs hardness 9
- No cleavage, though parting may be visible
- Commonly barrel-shaped, prismatic, or tabular crystals
- High density with glassy to adamantine luster

## Composition and classification

Corundum is aluminum oxide, Al₂O₃. Trace chromium produces ruby red, while iron, titanium, and other elements contribute blue, yellow, green, pink, and additional sapphire colors. Color alone cannot establish natural origin, treatment, or gem variety.

## Appearance

Corundum may form hexagonal barrel-shaped prisms, tabular crystals, rounded grains, or massive aggregates. Weathered crystals can look dull even though fresh surfaces are vitreous to adamantine.

Parting may create planar breaks that resemble cleavage, but true cleavage is absent. Some crystals display color zoning, silk-like inclusions, or a star effect when cut appropriately.

## Identification tests

- Hardness: 9. Scratches quartz readily; only diamond is harder among standard Mohs minerals.
- Streak: White or colorless. Not diagnostic by itself and destructive testing should be avoided on attractive material.
- Cleavage: None; parting may occur. Repeated true cleavage faces suggest another mineral.
- Density: High; specific gravity near 4. Heavier than quartz of equal size but requires careful comparison.

## Common lookalikes

- Quartz: Much softer at Mohs 7 and commonly forms six-sided prisms with pyramidal terminations.
- Garnet: Usually softer, commonly equant, and belongs to a mineral group with different crystal forms and density ranges.
- Spinel: Typically forms octahedra and is softer; gem material often requires optical or laboratory testing.

## Direct side-by-side comparisons

- Quartz vs. corundum — https://lithomap.app/resources/mineral-identification/compare/quartz-vs-corundum
- Kyanite vs. corundum — https://lithomap.app/resources/mineral-identification/compare/kyanite-vs-corundum

## Most relevant related guides

- Quartz — https://lithomap.app/resources/mineral-identification/quartz
- Kyanite — https://lithomap.app/resources/mineral-identification/kyanite
- Garnet — https://lithomap.app/resources/mineral-identification/garnet
- Apatite — https://lithomap.app/resources/mineral-identification/apatite

## Formation

Corundum forms in aluminum-rich, silica-poor metamorphic and igneous settings, including marbles, gneisses, schists, syenites, and some pegmatites.

Its hardness and chemical resistance allow crystals to survive weathering and accumulate in placer deposits with other dense minerals.

## Occurrence and geologic setting

Corundum is uncommon because silica-rich conditions favor other aluminum minerals. It occurs in selected metamorphic and igneous terranes and in sediment derived from them.

Field tests cannot establish whether colored corundum is natural, synthetic, treated, gem quality, or valuable.

## Uses and significance

Ruby and sapphire varieties are gemstones. Synthetic and natural corundum are also used as abrasives, refractories, bearings, watch windows, lasers, and durable optical components.

## Field safety and limitations

- Do not scratch, strike, or streak a potentially valuable crystal.
- Use a qualified gemologist or laboratory for origin and treatment questions.
- Verify collecting permission; placer access and mineral rights can differ from surface access.

## Authoritative sources

- Corundum and Its Occurrence and Distribution in the United States — U.S. Geological Survey: https://www.usgs.gov/publications/corundum-and-its-occurrence-and-distribution-united-states-a-revised-and-enlarged
- Minerals and their physical properties — U.S. National Park Service: https://www.nps.gov/subjects/geology/minerals.htm
- Mohs Hardness Scale — U.S. National Park Service: https://www.nps.gov/articles/mohs-hardness-scale.htm
- Natural Gemstones: Mineral Gemstones — U.S. Geological Survey: https://pubs.usgs.gov/gip/gemstones/mineral.html

## Specimen photograph

- Image: https://lithomap.app/_delivery/identification/minerals/corundum.webp
- Description: Two-centimeter corundum specimen showing hexagonal symmetry
- Creator: Nessa Eull
- License: CC0 1.0 — https://creativecommons.org/publicdomain/zero/1.0/
- Source record: https://commons.wikimedia.org/wiki/File:Corundum_(GeoDIL_number_-_1438).jpg

This reference supports preliminary field identification. It is not a laboratory determination, appraisal, hazard assessment, proof of origin, or statement of land ownership, legal access, or collecting permission.