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Chrome Tourmaline

Last updated: April 2026

Chrome tourmaline is colored by Cr³⁺ rather than iron, producing a rich emerald-like green. The Chelsea filter red reaction is the fastest field diagnostic. Primary sources are Tanzania, Kenya, and Pakistan. Rarer and more valuable than ordinary verdelite.

Chrome Tourmaline has a specific gravity of 3.10 and a refractive index of 1.624–1.660, with a Mohs hardness of 7.0–7.5. It is uniaxial −.

Physical & Optical Properties

RI Range1.624–1.660
SG Range3.00–3.20
SG Typical3.10
Hardness (Mohs)7–7.5
Crystal SystemTrigonal
Optic CharacterDR Uniaxial (−)
Birefringence0.018
Dispersion0.017
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterRed
PleochroismStrong Dichroic
ColorsGreen
SpeciesTourmaline
VarietyChromian Dravite or Chromian Elbaite
Green

Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.

Through the refractometer

Chrome Tourmaline varies 1.624 – 1.660 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORotate the stone through a full 360°, reading both edges atevery 45°. SEETwo boundaries, 0.018 apart at their widest separation. Thepair drawn here is one specimen: this species is publishedat 1.624 – 1.660, and the two edges slide together anywhereacross it, between the dashed marks. MEANSThat gap IS the birefringence — and it is the stable part.Where the pair sits varies by 2 printed divisions from stoneto stone, so the gap identifies and the position onlynarrows. Take max minus min across the whole rotation; asingle position understates it. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

Through the polariscope

Chrome Tourmaline Blinks 4× per turn (DR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Chrome Tourmaline blinks light-dark four times in a full turn. That is doubly refractive. It does NOT tell you uniaxial from biaxial — that is the optic figure, which needs a conoscope. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

Chrome Tourmaline SG 3.00–3.20 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 17 other species in the table overlap this range, so a reading here narrows the field to 18 rather than naming the stone. Combine it with a refractive index to separate what is left. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

Chrome Tourmaline Two colours (dichroic) BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Chrome Tourmaline shows dark green in one window and brownish-green in the other. Rotate the instrument 45° and the two windows become IDENTICAL — the null. So "both windows the same" means either the stone is singly refractive or you are holding the instrument at the wrong angle, and the only way to tell is to turn it. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

Green TourmalineIndicoliteParaiba TourmalineRubelliteTourmalineWatermelon Tourmaline

Key Differentiators

Common Simulants

Treatments

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a chrome tourmaline? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Chrome Tourmaline is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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