Iolite
Last updated: August 2026
Famous for its extreme pleochroism, showing different colors when viewed from different directions.
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
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).
Price Context
Natural — low ($/ct)$20
Natural — high ($/ct)$100
NotePer carat; undervalued gem; strong pleochroism is the key diagnostic feature
Price context is approximate. Loupewise is not an appraisal tool. Results are indicators, not certified valuations.
Related Comparisons
Measurement Guides
Identifying an iolite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Iolite is one of the 130 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →