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Pleochroism Score

Score pleochroism strength from absorption coefficients at two axes. Free pleochroism score tool for pleochroism, score, and more.

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Inputs that matter
Absorption Axis 1, Absorption Axis 2
Output to expect
Pleochroism Score
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Pleochroism Grading Scale

Gemological institutes score pleochroism on a 0–3 scale: 0 = none, 1 = weak (barely visible), 2 = moderate (easily visible), 3 = strong (distinctly different colors). Observation uses a calcite dichroscope.

Score: 0 (none) → 1 (weak) → 2 (moderate) → 3 (strong)

Absorption Coefficient Method

Quantitative pleochroism compares absorption coefficients (α and γ) along orthogonal crystallographic axes. Greater difference indicates stronger pleochroism.

Pleochroism ∝ |α - γ| / mean(α, γ)

Cutting Orientation Impact

For strongly pleochroic gems, table orientation along the optic axis minimizes color zoning. Misorientation in tanzanite, iolite, or kunzite visibly degrades face-up color.

Optimal cut: table ⊥ optic axis for strongest axis color

Updated: July 2026

Example Scenarios

A tanzanite crystal showing blue, violet, and burgundy along three axes under dichroscope.

Axis 1: Blue-violetAxis 2: VioletAxis 3: Burgundy/redCrystal System: Orthorhombic (trichroic)

→ Pleochroism score: 3 (strong); cut with table perpendicular to blue axis

A blue sapphire showing slight color variation between two axes.

Axis 1: Deep blueAxis 2: Slightly greenish blueCrystal System: Trigonal (dichroic)

→ Pleochroism score: 1 (weak); orientation less critical for face-up color

Iolite (cordierite) showing violet-blue vs near-colorless/watery axes.

Axis 1: Violet-blueAxis 2: Pale yellow/wateryCommon Name: 'Water sapphire'

→ Pleochroism score: 3 (strong); improper orientation produces washed-out face-up color

Common Mistakes to Avoid

Observing pleochroism without a dichroscope

Naked-eye color variation can result from color zoning, not true pleochroism. Always confirm with a calcite dichroscope showing two distinct windows side by side.

Confusing pleochroism with color change (alexandrite effect)

Pleochroism shows different colors along crystal axes under the same light source. Color change (photochromism) shows different colors under different light sources (incandescent vs daylight).

Scoring trichroic gems on only two axes

Orthorhombic, triclinic, and monoclinic gems may show three distinct colors. Rotate the stone through all axes and report the strongest and weakest colors.

FAQ

Iolite, tanzanite, andalusite, kunzite, and bi-color tourmaline show strong (3) pleochroism. Ruby and sapphire show weak (1) pleochroism. Diamond and garnet are isotropic and show none (0).

For strongly pleochroic gems, the cutter orients the table to display the most desirable color axis face-up. In tanzanite, this means cutting with the blue axis perpendicular to the table, not the burgundy axis.

Dichroism shows two colors (two axes) in uniaxial and some biaxial crystals. Trichroism shows three colors along three axes in biaxial crystals (orthorhombic, monoclinic, triclinic systems).

Yes. Pleochroism is a key diagnostic property. Strong blue/violet pleochroism suggests tanzanite or iolite; strong yellow/green pleochroism suggests peridot; absence of pleochroism in a colored stone suggests garnet or spinel.

Place the dichroscope eye-close to the stone with a strong light source behind the gem. Rotate the stone slowly and observe both calcite windows. Different colors in each window confirm pleochroism; identical colors indicate none.

About Pleochroism Score

Pleochroism is the appearance of different colors when a gemstone is viewed from different crystallographic directions. Gemologists grade pleochroism strength using a dichroscope to aid identification and assess how cutting orientation affects the finished stone's color.