Kyanite
KyaniteJewelryWiki Editorial·2 sources·Reviewed 2026-08-04
A blue bladed stone with a property almost no other gem has: its hardness depends on the direction you scratch it. IGS gives the range as 4 to 7.5 within a single crystal — soft along the blade, hard across it. That makes it hard to cut, and it means how a stone was oriented by the cutter decides how it survives being worn.
Properties
| Property | Value | Authority |
|---|---|---|
| Mohs hardness | 4–7.5; varies with direction in single crystals. This is not a range across specimens like most hardness figures — it is a range WITHIN one stone, along versus across the blade. [1] | — |
| Specific gravity | 3.53–3.68; gems usually at the upper end [1] | — |
| Chemical formula | Al2SiO5 The same formula as andalusite — the two are polymorphs, identical in chemistry and different in structure, which is why they look nothing alike. [2] | — |
| Crystal system | Triclinic [2] | — |
| Common treatments | Heat treatment, per IGS. [1] | — |
| Suitability for daily wear | Depends on the cut, which is unusual enough to be worth saying plainly. Kyanite also has perfect cleavage, so a knock is a greater risk than a scratch whichever way the stone was oriented. [1] | — |
| Care cautions | Avoid ultrasonic and steam cleaners — the cleavage makes vibration a real hazard. [1] | — |
Sources
- International Gem Society — gemstone articles — International Gem Society (IGS, gemstone information articles (gemsociety.org))
- Handbook of Mineralogy — Mineral Data Publishing / Mineralogical Society of America (Handbook of Mineralogy, Mineral Data Publishing)