Pleochroism

 

Pleochroism means “many coloured”. The “apparent” colour of the gemstone is altered through pseudochromatic coloration. This pseudochromatic coloration is the appearance of “colour” that is not caused by any actual colour in the mineral but from varying optical effects created by spectral dispersion and double refraction.

Pleochroism is observable in plane-polarized light. It is caused by the ability of some minerals (for instance double-refractive minerals such as Alexandrite) to absorb particular wavelengths of light. Because of the selective absorption of certain wavelengths of light, the transmitted light appears coloured and the minerals consequently seem to be a different colour, depending on the direction in which they are looked at (The direction of vibration of the waves that cross them: as light emerges from a doubly refracting transparent stone, the beam is split into two polarized light rays, each vibrating in planes at right angles to each other. If the light beam emerges from a coloured stone, each new refractive indices may undergo a different degree of colour absorption.) The colour depends on the thickness and the chemical and crystallographic nature of the mineral.

 Pleochroic colours are observed and recorded in the mineral data and may be used to identify the particular mineral.

Gana Moonfire

 

 

 

 

Sources

http://edafologia.ugr.es/OptMine/ppl/pleow.htm

http://webmineral.com/help/Pleochroism.shtml

http://www.allaboutgemstones.com/gemstone_pleochroism.html

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