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Kyanite

Last updated: August 2026

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.

Kyanite has a specific gravity of 3.68 and a refractive index of 1.712–1.734, with a Mohs hardness of 4.5–7.0. It is biaxial −.

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 (−)
Sources differ — GIA / FEEG: biaxial negative (B−) · Gem-A (FGA): biaxial positive (B+). Kyanite is optically biaxial negative (2V 78–83°) per GIA and standard mineralogy — this is what the app uses. Note that the Gem-A syllabus constants datasheet prints it as biaxial positive (B+); FGA candidates may be graded against that sheet.
Birefringence0.016
Dispersion0.020
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismStrong Trichroic
ColorsBlue Violet, Colorless, Green
SpeciesKyanite
Blue VioletColorlessGreen

Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.

Through the refractometer

Kyanite reads 1.715 – 1.731 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORotate the stone through a full 360°, reading both edges atevery 45°. SEETwo boundaries, 0.016 apart at their widest separation. MEANSThat gap IS the birefringence. Take max minus min across thewhole rotation — a single position understates it, so onereading is an incomplete one. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

Key Differentiators

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a kyanite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Kyanite is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

Practice exams →