Trichroism

Trichroism is the optical property of a material to display three distinct colors when viewed under polarized light from different crystallographic orientations. It is a type of pleochroism, the broader phenomenon in which a substance changes color depending on the direction of light polarization. Trichroism occurs in certain anisotropic crystals, particularly those with three principal optical directions, where light absorption varies along each direction and produces a different color as the crystal is rotated.

The effect arises from differential absorption of light along different axes of the crystal. In a polarized-light setup, each orientation highlights a different color because certain wavelengths are absorbed more strongly along one direction than another. The observed colors depend on the wavelength of light and the thickness of the sample, and are most pronounced in materials with strong anisotropy.

In gemology and mineralogy, trichroism is used as a diagnostic property to identify minerals and characterize gemstones. It is typically observed with a polarized light microscope or gem tester by rotating the sample between crossed polarizers. The phenomenon can assist in distinguishing minerals that have similar pleochroic behavior or in assessing the quality and cut of a gemstone.

Commonly cited examples include tourmaline, which can exhibit three colors, and Tanzanite, noted for its strong trichroism, displaying blue, violet, and burgundy tones under polarized light. The intensity and hue of trichroism vary with crystal quality, orientation, thickness, and the light source.

See also: dichroism, pleochroism. Trichroism is a specific instance of pleochroism characterized by three observable colors.