Kyanite
Blue aluminum silicate commonly confused with sapphire or tanzanite. Uniquely among gemstones, kyanite has direction-dependent hardness: 4.5 along the bladed crystal length (c-axis) and 6.5–7.0 across it. This anisotropic hardness is a diagnostic field indicator.
Physical & Optical Properties
RI Range1.712–1.734
SG Range3.53–3.68
SG Typical3.68
Hardness (Mohs)4.5–7
Crystal SystemTriclinic
Optic CharacterDR Biaxial (−)
Birefringence0.016
Dispersion0.020
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismStrong Trichroic
ColorsBlue Violet, Colorless, Green
SpeciesKyanite
Key Differentiators
- Variable hardness by direction — 4.5 along crystal length, 6.5–7.0 across (unique among gemstones)
- RI 1.710–1.734 — distinctly lower than sapphire (1.762–1.778)
- Strong trichroism: colorless / dark blue / violet-blue
- Bladed crystal habit, perfect cleavage in two directions
- SG 3.65–3.70 — heavier than tanzanite (3.35–3.38)
Price Context
Natural — low ($/ct)$10
Natural — high ($/ct)$200
NotePer carat; fine blue specimens command higher prices
Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.
About Kyanite
Blue aluminum silicate commonly confused with sapphire or tanzanite. Uniquely among gemstones, kyanite has direction-dependent hardness: 4.5 along the bladed crystal length (c-axis) and 6.5–7.0 across it. This anisotropic hardness is a diagnostic field indicator.
Identifying a kyanite? GemID walks through these tests in order — RI, SG, fluorescence, and more.
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