Pleochroism is the phenomenon in which the color of a mineral varies or changes with viewing orientation. This means that when light passes through doubly refractive gemstones, the light gets split into two rays which are polarized at right angles to each other and travel at differing velocities through the gemstone causing the variation in color. Pleochroic gems are separated into two sub-categories: di-chroic and tri-chroic. Di-chroic gems show different colors in 2 directions, and tri-chroic gems show different colors in 3 directions.
As many gemstones exhibit pleochrosism, gem cutters have to be particularly adept in determining the ideal color position for rough gems when cutting to ensure that the finished gem exhibits the most desirable color through the table facet.
The use of a dichroscope helps identify the exact constituent colors seen from all directions. This allows the cutter to position the rough gems to exhibit the most valuable color.