Pronunciation: C:/Program Files/Git/ˌpliːəˈkroʊɪzəm/
Also called: trichroism, dichroism

Pleochroism /ˌpliːəˈkroʊɪzəm/ — the appearance of different colors when an anisotropic crystal is viewed along different optical axes under the same light. A property of the crystal’s internal symmetry, not of the illuminant. Distinct from color change, which is illuminant-driven.

The two-color and three-color cases

Dichroism — pleochroism in two distinct colors. Occurs in uniaxial crystal systems (tetragonal, hexagonal, trigonal). The two colors correspond to the ordinary and extraordinary rays.
Trichroism — pleochroism in three distinct colors. Occurs in biaxial crystal systems (orthorhombic, monoclinic, triclinic). The three colors correspond to the three optical axes.

Cubic crystals (isotropic) cannot show pleochroism — diamond, garnet, and spinel transmit all directions equivalently.

Notable pleochroic gems

Tanzanite — strong trichroism: blue, violet, burgundy. The orientation chosen by the cutter determines the dominant face-up color.
Andalusite — strong trichroism: green, red, yellow. Often deliberately cut to display all three.
Iolite — strong trichroism: blue, gray, yellow. Historic name “water sapphire” reflects the intense blue along one axis.
Color-change diaspore (Zultanite) — strong trichroism in manganese-bearing varieties: violet-blue, pale green, rose to dark red.
Alexandrite — strong trichroism: green, orange, red.

Lapidary implications

For pleochroic gems, cutting orientation determines what color the buyer sees face-up. A tanzanite cut to display blue along the table will face up blue; the same crystal cut to display violet would face up violet. This is one of the calculations a cutter makes when planning the geometry of a faceted stone, balancing color preference against weight retention.

Pleochroism and color change together

The two phenomena can coexist. Zultanite is both pleochroic and color-changing. Pleochroism describes how the gem looks from different angles under one light source; color change describes how the gem looks from one angle under different light sources. The two are independent and combine multiplicatively in observation.

See also: color change.