As pleochroism ( trichroism ) is called in mineralogy, the " multi-color " (from the Greek: πλέων = Pleon for ' more ' and χρῶμα = Chroma for ' color' ) of minerals when viewed from different directions. The multi-color manifests itself either in a complete color change ( alexandrite - green in daylight, red in artificial light ) or in a different color ( emerald - green, blue and blue-green to yellow-green).
Pleochroism occurs in transparent, colored stones as well as on opaque. The reason for this is the uneven absorption of the light depends on the propagation direction and the polarization direction.
The dichroism is a special form of pleochroism with exactly two different colors.
The pleochroic properties of a mineral can be determined with the aid of a Dichroskops and used for determination and testing especially with semi-precious stones. Its effect on the color depth is mainly for lapidaries of importance in the selection of the cut, to avoid too dark or light ( pale ) colors.
Theoretical foundations and justifications for whether such an effect can occur are dealt with in a section of the theoretical crystallography.
Alexandrite, Hiddenite, Kunzite, ruby, sapphire and tourmaline can already see with the naked eye, the multi-color. Other examples are: