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
Amethyst vs Iolite: where the ranges part Where the ranges part Amethyst and Iolite, each recorded property on an axis scaled to the pair Amethyst Iolite Refractive index overlaps ranges overlap from 1.544 to 1.553 — a reading there decides nothing 1.544–1.553 1.540–1.560 1.535 1.550 1.565 Specific gravity overlaps ranges overlap from 2.63 to 2.65 — a reading there decides nothing 2.63–2.65 2.57–2.66 2.57 2.62 2.66 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 Amethyst 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. 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.
  2. 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.
  3. 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.
  4. 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

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? Loupewise walks you through dichroscope and polariscope tests step by step.

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Studying for a gemology practical? Loupewise’s timed exam simulators drill exactly this call — separating look-alike stones by their measured properties.

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