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Pleochroism


Pleochroismus Alexandrit In daylight, alexandrite is green and in artificial light it is red.

Pleochroism – multicolored gemstones

The technical term pleochroism, used in mineralogy, comes from the Greek and is made up of the two words pléon (more) and chroma (color). It is used for gemstones that exhibit a color change. A distinction is made between the case where the gemstone changes color completely and the case where only the color nuance changes.
An example of a complete color change is alexandrite, which is green to bluish green in daylight, but changes to an intense ruby red under artificial lighting. A representative of the second group is the emerald, which appears yellow-green, green, blue-green or blue depending on the direction of view.

Reasons for Pleochroism

Pleochroism occurs when the light rays entering the gemstone are refracted twice. Depending on the crystal structure, the light is split into two, three or more colors. In this context, we speak of dichroism, trichroism and polychroism.
A gem cutter must take the pleochroism of the gemstones into account, as this is the only way to avoid the gemstone having areas that are too pale or too dark. A special device, the dichroscope, has been developed to measure pleochroism.
In the case of gemstones, the gradations weak, clear and strong indicate how strong the pleochroism is.

Minerals with strong pleochroism

Andalusite is known for its striking pleochroism, which varies between the three colors yellow, greenish and dark red. Pleochroism is similarly striking in the mineral benitoite, which can appear colorless and blue. A strong color change can also be observed in malachite (transparent, greenish yellow, green) and tanzanite (purple red, dark blue, yellow-brown).

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