Gem library
Kyanite
The blade with two hardnesses
Kyanite is the only common stone whose hardness changes with the direction you scratch it: 4.5 along the crystal, 7 across it. That is not a laboratory curiosity, it is the first identification test, and it is what makes the stone a nightmare to cut.
- Hardness (Mohs)
- 4.5 – 5 (along the blade) · 6.5 – 7 (across it)
- Density
- 3.53 – 3.70
- Crystal system
- Triclinic
- Refractive index
- 1.710 – 1.734
- Cleavage
- Perfect on {100}
Mineralogical identity
- Family
- Silicates, nesosilicates
- Species
- Kyanite (disthene)
- Variety
- Blue kyanite
- Chemical formula
- Al₂SiO₅
- Crystal system
- Triclinic
- Classification
- Semi-precious stone
- Rarity
- Common
- Indicative price
- €1 – 60/ct
Etymology
From the Greek kyanos, «dark blue». Its other name, disthene, comes from dis-sthenos, «two strengths», and says exactly what sets it apart: two hardnesses in one crystal.
Physical & optical properties
- Hardness (Mohs)
- 4.5 – 5 (along the blade) · 6.5 – 7 (across it)
- Density
- 3.53 – 3.70
- Refractive index
- 1.710 – 1.734
- Birefringence
- 0.012 – 0.017
- Cleavage
- Perfect on {100}, distinct on {010}
- Fracture
- Splintery
- Lustre
- Vitreous to pearly
- Diaphaneity
- Transparent to translucent
- UV fluorescence
- Weak
- Pleochroism
- Distinct
- Colours
- blue, sky blue, green, black, colourless
Formation, geology & history
Formation and geology
Kyanite forms when aluminous rocks are metamorphosed under high pressure and moderate temperature, typically in the schists and gneisses that make up the roots of mountain belts. It shares its exact formula, Al₂SiO₅, with two other minerals: andalusite and sillimanite. The three are polymorphs, the same chemistry in three different architectures, and each forms in its own window of pressure and temperature. A geologist who finds kyanite in a rock knows at once that it was deeply buried: the mineral works as a field barometer. The finest blue blades from Nepal come from Himalayan quartzites, carried tens of kilometres down by the collision of India with Asia, then brought back up by erosion.
The double hardness, and what it demands
Kyanite's atomic bonds are far stronger across the length of the crystal than along it. A steel point scratches a blade lengthwise without effort and slides across it. The consequence for cutting is immediate: the lapidary cannot polish the stone with a single pressure and a single abrasive, and has to change the angle of attack midway. Add perfect cleavage in two directions and you see why faceted kyanite stays rare and expensive while the rough is common. Most stones on the market are polished blades or cabochons, not cut gems.
Colour and uses
The blue comes from iron and titanium, in proportions that take the stone from a pale, almost grey blue to a deep sapphire tone, often banded along the blade. Green kyanite, rarer, owes its colour to vanadium and comes mainly from Kenya and Tanzania. A black kyanite also exists, loaded with graphite inclusions. Away from jewellery, kyanite is a first-rank industrial mineral: heated, it converts to mullite, the refractory phase in the ceramics that line steel furnaces and spark plugs. By tonnage, that is by far its main outlet.
How to recognise it
- The two-hardness testThe decisive test, and it costs nothing. Scratch a blade along its length with a steel point: it marks. Scratch the same blade across: nothing. No other common blue mineral behaves this way.
- The bladed habitKyanite crystallises as flattened, elongated laths, often slightly curved or fanned out. A stubby, equidimensional crystal is not kyanite.
- Distinct pleochroismIn a dichroscope the stone shifts from deep blue to a very pale, almost colourless blue depending on direction. The change is visible to the naked eye by turning a blade in front of a window.
Imitations, treatments & confusions
- Blue glassOn cheap beads. It has neither the bladed habit, nor the cleavage, nor the double hardness, and shows round bubbles under a loupe.
- Dyed quartzSometimes sold as «blue kyanite» in sticks. The dye concentrates in the fractures, and quartz is hardness 7 in every direction.
- An honest mix-up with sillimaniteSame formula, same family, but sillimanite is fibrous and has a constant hardness of 6.5 to 7.5. This is not a fraud, it is a frequent dealer error.
Deposits & origins
Nepal (deep blue blades from the Annapurnas), Brazil (Minas Gerais), Switzerland (Pizzo Forno, the historic locality), India, United States (North Carolina), Russia, Austria, Kenya (green kyanite)
Care & precautions
- Warm soapy water and a soft cloth, rinsed quickly. The stone is not water-sensitive.
- Never ultrasonic or steam cleaning: the perfect double cleavage gives way under vibration.
- Set it as a pendant or earrings rather than a ring: a blade on a finger eventually splits.
- Store flat and separately, blades snap under sideways pressure.
Traditions & crystal healing
- Chakras
- Throat · third eye
- Zodiac signs
- Aries · Taurus · Libra
- Element
- Air
Tradition casts it as a stone of alignment and honest speech: it is laid on the throat for what will not come out. It has the rare reputation of never needing cleansing, which practitioners put down to a bladed structure that «holds nothing». Cultural heritage, never medicine.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Why is faceted kyanite so rare?
Because of the perfect double cleavage and the variable hardness: the stone splits during cutting, and polishing demands changing angles. The yield is low and the risk high, so few lapidaries attempt it.
Kyanite or disthene, which is correct?
Both. «Disthene» is the historic name, still dominant in continental mineralogy; «kyanite» rules the gem trade. Exactly the same mineral.
Can it be worn daily?
As a pendant, yes. As a ring, no: hardness 4.5 in one direction and perfect cleavage will not survive it. This is a stone to set, not a stone to knock.
Tools for this stone
Kyanite: only the tools this stone actually concerns, each carrying the value read on this page.