Kyanite
Al₂SiO₅ · Triclinic
Kyanite is the mineral that breaks the Mohs scale, because its hardness depends on the direction you test. It forms flat blue blades in high-pressure metamorphic rock and is one of the few minerals a beginner can identify with total confidence using nothing but a pocket knife.
Properties
- Mohs hardness
- 4.5–7Hardness 4.5–5 along the length of the blade and 6.5–7 across it. This directional difference is unique among common minerals and is diagnostic on its own.
- Streak
- white, colorless
- Lustre
- vitreous, pearly
- Colour
- Characteristically a streaky sky blue, often deeper along the centre of the blade.
- Cleavage
- Perfect in one direction along the blade.
- Fracture
- Splintery
- Specific gravity
- 3.5–3.7 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm kyanite
- Scratch along the blade with a steel knife — it marks. Scratch across the blade — it does not. Nothing else common behaves this way.
- Long, flat, bladed crystals, usually in a quartz or mica matrix.
- Streaky uneven blue colouring.
What kyanite gets confused with
Each of these is settled by one test. Run that test first.
Tourmaline is uniformly hard in every direction and has no cleavage.
Sillimanite
Sillimanite is fibrous and uniformly hard; kyanite is bladed and directionally soft.
Where it is found
Aluminium-rich schists and gneisses, and in quartz veins cutting them.
Is it worth anything?
Low, though sharp blue blades and rare gem-quality material have collector value.