metamorphic mineral
Kyanite (Blue or Blue-Grey)
Kyanite (Al2SiO5)
Hardness: variable 4.5 to 7 on Mohs scale (anisotropic); Color: blue, blue-grey, green; Luster: Vitreous to pearly; Crystal structure: Triclinic (bladed/columnar); Cleavage: Perfect in one direction.
- Hardness
- variable 4
- Color
- blue, blue-grey, green
- Luster
- Vitreous to pearly
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Physical properties
Hardness: variable 4.5 to 7 on Mohs scale (anisotropic); Color: blue, blue-grey, green; Luster: Vitreous to pearly; Crystal structure: Triclinic (bladed/columnar); Cleavage: Perfect in one direction.
Formation & geological history
Formed at high pressure and relatively low temperatures during the regional metamorphism of clay-rich (pelitic) sedimentary rocks. Common in schists and gneisses.
Uses & applications
Used industrially for manufacturing heat-resistant ceramics (high-alumina refractories), spark plugs, and laboratory crucibles. High-quality specimens are used as gemstones and in collection.
Geological facts
Kyanite is unique because its hardness varies significantly depending on the direction of the crystal axis: it is about 4.5 to 5 parallel to the length of the crystal and 6.5 to 7 across it.
Field identification & locations
Identify by its characteristic bladed crystal habit and its unusual variable hardness. Commonly found in Brazil, Switzerland, and the USA (Appalachian Mountains). Look for flaky, elongated blue-tinted sticks in metamorphic outcrop.
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