Amethyst vs Iolite: How to Tell Them Apart
Last updated: August 2026
Amethyst (quartz) and iolite (cordierite) are both violet-blue, and this pair is a trap for anyone leaning on the refractometer: the RI ranges overlap, and so do the SG ranges. The separation is optical character. Iolite is famous for its extreme pleochroism — strong trichroism showing violet-blue, light blue, and yellow-gray in different directions — while amethyst is only weakly dichroic. A dichroscope makes the call in seconds; the polariscope (uniaxial vs biaxial) and spectroscope confirm it.
Property Comparison
| Property | Amethyst (Quartz) | Iolite (Cordierite) | Diagnostic? |
|---|---|---|---|
| Pleochroism | Weak dichroic (purplish-red / bluish-purple) | Strong trichroic (violet-blue / light blue / yellow-gray) | Yes |
| Optic Character | DR U+ (uniaxial positive; 'bull's eye' optic figure) | DR B- (biaxial negative) | Yes |
| RI | 1.544 – 1.553 | 1.54 – 1.56 — sources differ: GIA 1.542–1.551 · Gem-A (FGA) / FEEG 1.54–1.56 | No |
| SG | 2.63 – 2.65 | 2.57 – 2.66 | No |
| Hardness | 7 | 7 – 7.5 | No |
| Birefringence | 0.009 | 0.012 | No |
| Crystal System | Trigonal | Orthorhombic | No |
| Cleavage | None | Distinct | Clue |
| Spectroscope | Broad Fe³⁺ band at 550 nm | Strong Fe²⁺ bands at 645 / 593 / 585 nm | Clue |
| Fluorescence (LW) | Inert | Inert | No |
The Definitive Tests
- Dichroscope — count the colors. Iolite shows strong trichroism: violet-blue, light blue, and yellow-gray in different directions. Amethyst shows only weak dichroism (purplish-red / bluish-purple) — often hard to see at all. Rotate the stone through several orientations; the pale yellow-gray third direction is the giveaway. Trichroism is diagnostic for this pair.
- Polariscope — optic figure and sign. Amethyst is uniaxial positive and quartz shows the characteristic 'bull's eye' optic figure. Iolite is biaxial negative. Where the dichroscope result is ambiguous on a pale stone, the optic figure settles it.
- Spectroscope. Iolite shows a group of strong iron (Fe²⁺) absorption bands at 645, 593, and 585 nm — diagnostic for iolite. Amethyst shows only a broad, moderate band around 550 nm. A clear multi-band iron pattern in the yellow-to-red region points to iolite.
- Know why RI won't do it. Amethyst reads 1.544–1.553; iolite reads 1.54–1.56 (published ranges differ by exam body — GIA lists 1.542–1.551, Gem-A and FEEG use 1.54–1.56, because cordierite RI varies with Mg/Fe composition). The ranges sit on top of each other. Use the refractometer for the optic character, not the number.
Common Mistakes
- Calling it on the refractometer number. This pair is the textbook case of overlapping RI: a reading near 1.55 is consistent with both. The behavior of the shadow edges (uniaxial vs biaxial) is informative; the raw number is not.
- Expecting SG to help. Amethyst's 2.63–2.65 falls inside iolite's 2.57–2.66. Hydrostatic weighing cannot separate this pair.
- Confirming "pleochroism present" and stopping. Both stones are technically pleochroic. The separation is strength and count: iolite's trichroism is strong and obvious with a distinctly yellow-gray direction; amethyst's dichroism is weak and stays within purples. Look for the third color, not just a change.
- Studying one RI range for the exam. If you are preparing for a specific credential, learn the range your exam body publishes: GIA 1.542–1.551; Gem-A (FGA) and FEEG 1.54–1.56. Candidates should expect their own syllabus values on paper — and overlap with quartz either way.
Frequently Asked Questions
Can a refractometer separate amethyst from iolite?
Not reliably. Amethyst reads 1.544–1.553 and iolite 1.54–1.56 — the ranges overlap. The optic behavior on the instrument does help: amethyst is uniaxial positive while iolite is biaxial negative. But the fastest and most decisive test is the dichroscope, because iolite's trichroism is unmistakable.
What is the fastest test to tell iolite from amethyst?
The dichroscope. Iolite shows strong trichroism — violet-blue, light blue, and yellow-gray in different directions. Amethyst shows only weak dichroism (purplish-red / bluish-purple). The pale yellow-gray third direction is the giveaway: if rotating the stone produces a distinctly yellowish or gray face, it is not amethyst.
Why do published RI values for iolite differ between references?
Cordierite (iolite) RI varies with Mg/Fe composition, so published reference ranges differ by exam body: GIA lists 1.542–1.551 while Gem-A (FGA) and FEEG use 1.54–1.56. Either way the range sits on top of amethyst's 1.544–1.553, which is why RI is not the separating test for this pair.
Identifying a violet stone? GemID walks you through dichroscope and polariscope tests step by step.
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