Pseudochromatic Color: Pleochroism, Dichroism & Trichroism
Double Refraction & Color
Gemstones that are "doubly refractive" can exhibit pleochroism, dichroism or trichroism, all of which can alter the "apparent" color of the gemstone through pseudochromatic coloration. This pseudochromatic coloration is the appearance of "color" that is not caused by any actual color in the mineral (ie. idiochromatic coloration, allochromatic coloration), but from varying optical effects created by spectral dispersion and double refraction.
Pleochroism is only observed in doubly refractive (anisotropic or uniaxial minerals) colored gemstones, although some doubly refractive specimens may not show pleochroism. Dichroism is only observed in uniaxial minerals, while trichroism is only observed in biaxial minerals.
As light emerges from a doubly refracting transparent stone, the beam is split into two polarised light rays, each vibrating in planes at right angles to each other. If the light beam emerges from a colored stone, each new refractive indices may undergo a different degree of color absorption. Minerals in the isometric crystal system cannot exhibit any type of pleochroism.
Unlike pleochroism which is not dependent on the viewing angle, "dichroism" (from the Greek word dikhroos, meaning "two-colored") is when two colors are perceived when observing the stone from two different directions (unlike dispersion). Minerals in the hexagonal, tetragonal, and trigonal crystal system are dichroic, and can only show two colors. In the field of optics, dichroism is sometimes used to denote pleochroism as described above.
With trichroism, three colors are perceived when observing the stone from two different directions. Minerals in the monoclinic, orthorhombic, and triclinic crystal system are trichroic, and can show three colors.
In the field on mineralogy, pleochroism is helpful in determining the correct identification of gems that are otherwise very similar yet have different pleochroic color schemes. These refractive attributes are determined by using a dichroscope (polarizing petrographic microscope), which can also detect the optic axis direction.
Gems That Exhibit Pleochroism
Gems That Exhibit Dichroism
Gems That Exhibit Trichroism
Bibliography & Suggestions for Further Study on Pleochroism
1. ICA, About Colored Gemstones . The Colored Gem Association
2. JJKent, Inc., Absorption & Dichroism in Gems . www.jjkent.com
3. Wuerzburg Uniaxial Minerals . www.geographie.uni-wuerzburg.de
4. UC Berkeley, Color in Minerals . Berkeley.edu
5. Stanford University, Refraction in Gems . www.stanford.edu
6. Stephen A. Nelson Introduction to Uniaxial Minerals . www.tulane.edu
7. Kurt Nassau, The Origins of Color in Minerals . American Mineralogist
8. T. Loomis, P Stevens, Vibrational spectroscopy of minerals . Queensland University of Technology