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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)Quantitative pleochroism compares absorption coefficients (α and γ) along orthogonal crystallographic axes. Greater difference indicates stronger pleochroism.
Pleochroism ∝ |α - γ| / mean(α, γ)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 colorUpdated: July 2026
A tanzanite crystal showing blue, violet, and burgundy along three axes under dichroscope.
→ Pleochroism score: 3 (strong); cut with table perpendicular to blue axis
A blue sapphire showing slight color variation between two axes.
→ Pleochroism score: 1 (weak); orientation less critical for face-up color
Iolite (cordierite) showing violet-blue vs near-colorless/watery axes.
→ Pleochroism score: 3 (strong); improper orientation produces washed-out face-up color
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.
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).
Orthorhombic, triclinic, and monoclinic gems may show three distinct colors. Rotate the stone through all axes and report the strongest and weakest colors.
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.