Pleochroïsme

Pleochroïsme, or pleochroism in English, is an optical property of certain anisotropic materials whereby the material shows different colors when viewed in transmitted light from different directions. This effect arises from anisotropy in how the material absorbs light along its crystallographic axes.

In crystals, light experiences different absorption coefficients along different directions. When light is polarized, the portions aligned with each axis are absorbed differently, so the transmitted color depends on the orientation of the crystal relative to the polarized light. Uniaxial crystals produce two refracted rays with distinct absorptions; biaxial crystals can produce three different apparent colors. The overall result is that rotating the crystal or the polarizer causes a change in color or shade.

Pleochroïsme is usually categorized as dichroism (two colors) or trichroism (three colors). It is most commonly observed in minerals and in some gemstones under plane-polarized transmitted light, such as in petrographic microscopy or gemology. Observations are made with a polarized light source and an analyzer, often with rotating stage.

Applications include mineral identification, crystallography, and gem grading, where pleochroism helps distinguish minerals and assess crystal orientation. Notable examples include cordierite (iolite), which shows strong dichroism, and tourmaline, which can display pronounced two-color dichroism. Some gemstones also exhibit pleochroism and, in rare cases, color shifts when viewed from different directions. Pleochroïsme is related to but distinct from color changes caused by illumination spectrum, such as the alexandrite effect, which is primarily due to changes in light source rather than orientation.