Short answer: kyanite's hardness depends on direction. Along the length of a blade it is about 4.5 to 5 on the Mohs scale. Across the blade it is about 6.5 to 7. Few common minerals show a difference this large.
Why direction matters
Hardness is a measure of how strongly a mineral's atoms hold together when something tries to scratch it. In kyanite the bonds are not equally strong in every direction. Scratching along the blade works against the weaker ones. Scratching across it meets the stronger ones.
This is a property to know, not a test to run. Scratching a collector specimen damages it.
What kyanite is
Kyanite is an aluminum silicate. It forms long, flat, bladed crystals, most often blue, sometimes with green, gray, or white zones. The name comes from the Greek word for blue. The color is usually strongest down the center of a blade and paler at the edges.
How it forms
Kyanite is a metamorphic mineral. It grows when aluminum-rich rocks, such as former clays, are buried and squeezed under high pressure. Geologists use it as an indicator: finding kyanite in a rock says that rock was once deep in the crust.
It shares its chemistry with two other minerals, andalusite and sillimanite. All three are the same ingredients arranged differently, and which one forms depends on the pressure and temperature at the time.
Kyanite with quartz
Blue kyanite blades set in white or clear quartz are a classic combination. The two minerals grow together in veins within metamorphic rock, and the contrast between sharp blue blades and pale quartz is what makes these pieces display so well.
Handling and care
- Blades are brittle and can splinter along their length. Lift a specimen by the quartz or matrix, not by the blades.
- Dust with a soft, dry brush.
- Skip ultrasonic cleaners and anything abrasive.
What to look for
- Depth and evenness of blue along the blades.
- Intact blade tips and edges.
- How the blades sit in the quartz. Blades that stand clear of the matrix show better and are more exposed, so check condition closely.
- Weight and dimensions, and whether the photographs show the exact piece.
Browse kyanite specimens at Digital Towns Market
Digital Towns Market
Kyanite in stock
Sources
- Tulane University, Physical Properties of Minerals: https://www2.tulane.edu/~sanelson/eens211/physprop.htm
- Fort Hays State University, Sternberg Museum, mineral names and properties: https://sternberg.fhsu.edu/research-collections/geology/mineral-names-page.html