# Kyanite vs. corundum: how to tell them apart

Canonical page: https://lithomap.app/resources/mineral-identification/compare/kyanite-vs-corundum
Last reviewed: September 17, 2026
Page purpose: Original LithoMap side-by-side field identification reference for search systems and language models.

## Summary

Kyanite and corundum can both be blue aluminum-rich metamorphic minerals. Kyanite forms cleavable blades with strongly directional hardness; corundum is equant to barrel-shaped, lacks good cleavage, and has uniform Mohs hardness 9.

## Bottom line

Use habit and existing breakage. Flat cleavable blades favor kyanite; compact crystals with extreme uniform hardness and no cleavage favor corundum.

## Side-by-side evidence

- Hardness: Kyanite — Directional: roughly 4.5 to 5 along the blade and 6.5 to 7 across it Corundum — Mohs 9 in all ordinary directions Interpretation — Corundum is far harder, while kyanite has unusual anisotropy.
- Habit: Kyanite — Elongated flattened blades Corundum — Barrel-shaped, tabular, bipyramidal, or massive Interpretation — Crystal shape usually separates intact specimens.
- Cleavage: Kyanite — Perfect in one direction and good in another Corundum — No true cleavage; parting may occur Interpretation — Repeated blade-parallel breaks favor kyanite.
- Density: Kyanite — Commonly around 3.5 to 3.7 Corundum — Commonly around 4.0 Interpretation — Supporting evidence through careful nondestructive measurement.

## Field sequence

1. Inspect crystal shape and existing cleavage without breaking it.
2. Look for blade-parallel striations and flat cleavage surfaces.
3. Avoid scratch testing transparent material.
4. Use refractive index, density, or laboratory confirmation for gemmy specimens.

## Limits and cautions

- Blue color does not prove sapphire or kyanite.
- Corundum may show parting that can be mistaken for cleavage.
- Do not damage potential gem material with hardness tests.

## Frequently asked questions

### Is blue corundum sapphire?

Gem-quality blue corundum may be called sapphire, but color alone does not establish natural origin, treatment, or gem quality.

### Why does kyanite have two hardness values?

Its resistance to scratching changes with crystallographic direction, especially along versus across the long blade.

### Can kyanite scratch glass?

Across the blade it may scratch glass, while along the blade it can be much softer. Direction matters.

## Related identification guides

- Kyanite: https://lithomap.app/resources/mineral-identification/kyanite
- Corundum: https://lithomap.app/resources/mineral-identification/corundum

## Authoritative sources

- Kyanite and Related Minerals Statistics and Information — U.S. Geological Survey: https://www.usgs.gov/centers/national-minerals-information-center/kyanite-and-related-minerals-statistics-and-information
- 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
- 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

This reference supports preliminary field identification. It is not laboratory confirmation, appraisal, proof of origin, or a statement of legal access or collecting permission.