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.
The Definitive Tests
- 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.
- 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.
- 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
- Using trichroism as the identifier. Strong trichroism separates tanzanite from sapphire — but iolite is also strongly trichroic. If violet-blue plus trichroic is your whole test, iolite passes it. Confirm with RI or SG, which do not overlap.
- Confusing the two RI reference ranges for iolite. GIA publishes 1.542-1.551; Gem-A syllabus constants give 1.54-1.56. Both are correct for their exam body — iolite's RI genuinely varies with composition. Know which range your credential expects, and note that neither comes anywhere near tanzanite.
- Assuming similar durability and care. Tanzanite (hardness 6-7, perfect cleavage) needs gentler handling than iolite (hardness 7-7.5, distinct cleavage). This matters for setting and care advice rather than identification, but it is part of giving honest guidance on the cheaper substitute.
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? GemID walks you through refractometer, SG, and dichroscope tests step by step.
Open GemID →Studying for a gemology practical? The trichroic look-alikes are where memorized shortcuts fail. GemID’s timed exam simulators drill exactly this call — separating look-alike stones by their measured properties.
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