LithoMap

Mineral identification comparison

Kyanite vs. corundum: how to tell them apart

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.

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Intergrown bladed kyanite crystals about five centimeters across from Boehl’s Butte, Idaho
KyaniteAluminum silicate · Al₂SiO₅Photo: Shannon Heinle · CC0 1.0
Two-centimeter corundum specimen showing hexagonal symmetry
CorundumAluminum oxide · Al₂O₃Photo: Nessa Eull · CC0 1.0
Quick answer

The most useful distinction

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

Kyanite and Corundum comparison table

PropertyKyaniteCorundumHow to use it
HardnessDirectional: roughly 4.5 to 5 along the blade and 6.5 to 7 across itMohs 9 in all ordinary directionsCorundum is far harder, while kyanite has unusual anisotropy.
HabitElongated flattened bladesBarrel-shaped, tabular, bipyramidal, or massiveCrystal shape usually separates intact specimens.
CleavagePerfect in one direction and good in anotherNo true cleavage; parting may occurRepeated blade-parallel breaks favor kyanite.
DensityCommonly around 3.5 to 3.7Commonly around 4.0Supporting evidence through careful nondestructive measurement.
Field sequence

A practical identification sequence

  1. 1

    Inspect crystal shape and existing cleavage without breaking it.

  2. 2

    Look for blade-parallel striations and flat cleavage surfaces.

  3. 3

    Avoid scratch testing transparent material.

  4. 4

    Use refractive index, density, or laboratory confirmation for gemmy specimens.

Limits and cautions

Where the distinction gets difficult

  • 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.

Use several independent observations. This comparison supports preliminary field identification, not laboratory confirmation, appraisal, proof of origin, or collecting permission.

Common questions

Kyanite vs. corundum FAQ

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.

Source basis

Authoritative sources

LithoMap wrote this comparison in original language from the physical properties and geological concepts supported by these public scientific references.

Editorial review: September 17, 2026.

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