Tanzanite vs Iolite: How to Tell Them Apart

Last updated: August 2026

Iolite is often sold as a budget alternative to tanzanite, and the confusion runs both ways: both are violet-blue, both are orthorhombic, and — the trap — both are strongly trichroic. Candidates who learned "trichroism means tanzanite" from the sapphire vs tanzanite separation walk straight into this one. The measured properties leave no doubt: the RI and SG ranges do not come close to overlapping, and SG is the fastest separator.

Property Comparison

Property Tanzanite (Zoisite) Iolite (Cordierite) Diagnostic?
RI 1.691 – 1.700 1.540 – 1.560* Yes
SG 3.35 – 3.38 2.57 – 2.66 Yes
Hardness 6 – 7 7 – 7.5 No
Crystal System Orthorhombic Orthorhombic No
Optic Character DR B+ (biaxial positive) DR B− (biaxial negative) No
Birefringence 0.009 0.012 No
Fluorescence (LW) Weak red-brown Inert No
Cleavage Perfect Distinct No
Pleochroism Strong trichroic (blue / violet / burgundy) Strong trichroic (violet-blue / light blue / yellow-gray) Yes

* Sources differ on iolite’s RI — 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. Either way, every published iolite range sits far below tanzanite’s 1.691–1.700.

Tanzanite vs Iolite: where the ranges part Where the ranges part Tanzanite and Iolite, each recorded property on an axis scaled to the pair Tanzanite Iolite Refractive index separates no overlap — a gap of 0.131, wider than the ±0.010 a reading is trusted to 1.691–1.700 1.540–1.560 1.533 1.620 1.707 Specific gravity separates no overlap — a gap of 0.69, wider than the ±0.10 a reading is trusted to 3.35–3.38 2.57–2.66 2.53 2.97 3.42 Birefringence too close to call a gap of 0.003, within the ±0.003 the engine allows a reading — too close to call 0.009 0.012 0.006 0.010 0.015 Ranges as recorded in gems.json for Tanzanite and Iolite; every axis is scaled to the pair, so read the tick labels. A gap wider than the instrument's tolerance (RI ±0.010, SG ±0.10, birefringence ±0.003, as the identification engine applies them) means a single clean reading settles it; an overlap, or a gap inside that tolerance, means that instrument cannot, on its own. Hardness is not drawn: a scratch test damages a cut stone. Birefringence is a single recorded value per material.

The Definitive Tests

  1. Heft / SG measurement — the fastest separator. Tanzanite's SG is 3.35-3.38; iolite's is 2.57-2.66. Per the reference data, SG is the fastest way to tell them apart: a hydrostatic reading anywhere near 3.35 rules out iolite immediately, and the density difference is large enough to notice as heft in loose stones of similar size.
  2. Refractometer — RI reading. Tanzanite reads 1.691-1.700. Iolite reads in the mid-1.5s (1.540-1.560 on the Gem-A range; 1.542-1.551 per GIA). The gap is over 0.13 — enormous by refractometer standards. A single shadow edge settles it.
  3. Dichroscope — read the colors, not just the count. Both stones are strongly trichroic, so “three colors” proves nothing here. The colors themselves differ: tanzanite shows blue / violet / burgundy, while iolite shows violet-blue / light blue / yellow-gray. Iolite's washed-out yellow-gray third direction looks nothing like tanzanite's burgundy.

Common Mistakes

Frequently Asked Questions

Is iolite a real substitute for tanzanite?

Iolite is a natural gem with its own identity, commonly offered as a lower-cost alternative to tanzanite because both show violet-blue color with strong trichroism. Gemologically they are easy to separate: tanzanite's RI is 1.691-1.700 and SG 3.35-3.38, while iolite reads in the mid-1.5s with SG 2.57-2.66.

What is the fastest test to tell tanzanite from iolite?

Specific gravity. Tanzanite (SG 3.35-3.38) is much denser than iolite (SG 2.57-2.66). A refractometer reading is equally decisive: 1.691-1.700 for tanzanite versus the mid-1.5s for iolite.

Why do published RI values for iolite differ between sources?

Cordierite (iolite) RI varies with its magnesium-iron composition, so exam bodies publish different reference ranges: GIA lists 1.542-1.551, while the Gem-A syllabus constants give 1.54-1.56. Both are legitimate; check which your exam body uses.

Identifying a violet-blue stone? Loupewise walks you through refractometer, SG, and dichroscope tests step by step.

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Studying for a gemology practical? The trichroic look-alikes are where memorized shortcuts fail. Loupewise’s timed exam simulators drill exactly this call — separating look-alike stones by their measured properties.

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