pleochroic

Pleochroism is the property of certain materials to appear as different colors when viewed from different crystallographic directions, or when observed through polarized light. The effect arises from anisotropic absorption of light: the material absorbs different wavelengths along distinct optical axes, so transmitted light has different colors depending on orientation. Most commonly, minerals and gemstones exhibit pleochroism, which can be described as dichroism (two colors) or trichroism (three colors). In uniaxial crystals, two principal directions yield two colors; in biaxial crystals, up to three colors may be seen.

Observation is typically made in a petrographic microscope with polarized light. Rotating the stage or changing polarizers allows the observer to see color changes as the crystal is viewed along different axes. The precise colors and their visibility depend on crystal chemistry, thickness, and the orientation of the crystal relative to the light.

Causes include differences in absorption coefficients along axes caused by crystal structure and the presence of transition-metal ions that affect electronic transitions. Pleochroism is related to birefringence but is specifically a property of differential absorption, while birefringence concerns phase velocity differences.

Applications: Pleochroism is a diagnostic property used in mineral identification and gemology. Well-known pleochroic minerals include iolite (often cited for strong dichroism) and tourmaline, whose crystals can show notable color changes depending on viewing direction.

In all cases, pleochroism reflects the anisotropic optical properties of a material and provides information about its composition and crystal structure.