silicates
Kyanite
Al2OSiO4
- Also known as
- disthenecyanite
- Crystal system
- triclinic
- Hardness
- 4.5–7 on the Mohs scale
- Lustre
- vitreous
- Colour
- Blue, and also white, green, grey, black
- Where it forms
- In aluminium-rich metamorphic rocks at high pressure. Kyanite, andalusite and sillimanite have identical chemistry and different structures, and which one forms tells a geologist the pressure and temperature the rock experienced.
- Chiefly from
- NepalBrazilSwitzerlandNorth Carolina, United States
Why it is that colour
Iron and titanium, in the same charge-transfer arrangement that colours sapphire blue.
Safety
Safe to handle. Its bladed crystals are brittle along their length and it chips easily.
Where it comes from
Mined industrially for refractory ceramics, since it expands rather than shrinking when fired. Specimen material largely from Brazil and Nepal.
Historical use
Used in the linings of furnaces and kilns and in spark plugs, because heating converts it to mullite with an expansion that keeps the refractory tight rather than cracking.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
No traditional history. Named in 1789 from the Greek for dark blue. Its alternative name disthene, meaning two strengths, refers to its most unusual property. Modern meanings are recent.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Kyanite has a genuinely odd property that its old name records: its hardness depends on direction. Along the length of a blade it is about 4.5 and can be scratched with a knife; across the blade it is about 7 and cannot. Disthene means two strengths, and it is one of very few minerals where the Mohs number has to be given as a pair.
That is a useful corrective to the way hardness is usually discussed. A crystal is not a uniform lump; it is a directional structure, and its properties can differ along different axes. Hardness, cleavage, refractive index and thermal conductivity all can.
Identity checked against IMA-CNMNC master list, June 2025.