Kyanite — Properties, Tests & Identification

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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

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.

Measurement Guides

Identifying a kyanite? GemID 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 130 species in GemID’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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