Pleochroism

Pleochroism is the direction-dependent absorption of light in an anisotropic crystal, causing its apparent color to change with viewing direction or polarization.

Optical anisotropy: A material property in which optical behavior depends on direction. Pleochroism occurs when a crystal’s optical response varies with direction.
Crystal system: A classification of crystals by their symmetry and lattice geometry. Crystal symmetry constrains the number of distinct optical directions.
Polarizing microscope: A microscope equipped with polarizers for examining optical properties of specimens. Rotating a crystal under polarized light reveals changes in its absorbed colors.
Color change (gemology): A gemstone’s apparent color shift under different illumination sources. Unlike pleochroism, this effect depends mainly on the light source, not viewing direction.
Linear polarization: A state of light in which the electric field oscillates along a fixed direction. Crystal absorption depends on how the light’s polarization aligns with optical directions.
Anisotropy: The dependence of a material’s properties on direction. Directional dependence is the underlying condition that permits pleochroism.
Mineral identification: The determination of a mineral’s identity from its observable and measurable properties. Pleochroism is a diagnostic optical property used alongside other mineral tests.
Iridescence: A color effect produced by wavelength-dependent interference or diffraction from structure. Iridescence arises from light interference, whereas pleochroism arises from selective absorption.
Dichroism: A difference in absorption between two polarization directions or wavelengths. This two-direction form of selective absorption produces pleochroic color changes.
Electromagnetic wave: A propagating disturbance of electric and magnetic fields that carries energy. Light’s electric field interacts differently with different crystal directions.

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