Iolite: Specific Gravity 2.59, Refractive Index 1.540

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Iolite has a specific gravity of 2.59 and a refractive index of 1.540–1.560, with a Mohs hardness of 7.0–7.5. It is biaxial −.

Physical & Optical Properties

RI Range1.540–1.560

Sources differ — GIA: 1.542–1.551 · Gem-A (FGA) / FEEG: 1.54–1.56. Cordierite (iolite) RI varies with Mg/Fe composition, so published reference ranges differ by exam body. The app uses the Gem-A syllabus range for identification; GIA candidates should expect GIA's narrower nominal range.

SG Range2.57–2.66

SG Typical2.59

Hardness (Mohs)7–7.5

Crystal SystemOrthorhombic

Optic CharacterDR Biaxial (−)

Birefringence0.012

Dispersion0.017

Fluorescence LWInert

Fluorescence SWInert

Chelsea FilterInert

PleochroismStrong Trichroic

ColorsBlue Violet, Purple

SpeciesCordierite

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

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

Key Differentiators

Extreme pleochroism
RI near quartz but DR
Often has inclusions

Common Simulants

Blue sapphire: Sapphire: uniaxial negative; RI 1.762–1.770; SG 4.00; dichroic (not trichroic); much harder (9 Mohs).
Tanzanite: Tanzanite: biaxial positive; RI 1.691–1.700; SG 3.35; trichroism blue-violet-burgundy; softer (6.5 Mohs).

Commonly Confused With

Commonly confused with: Sapphire, Tanzanite.

Related Comparisons

Measurement Guides

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

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

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