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Derived    from  the  Greek  word  “pleion”,  “more”,  and  “chros”,  “colour”,  pleochroism  is an optical effect where different colours can  be  seen  in  a  gemstone  when  observed  from  different angles. Depending on the colours and  the attractiveness of the pleochroism, gems are  cut either to maximise or hide the effect. 

As  light  is  made  up  of  different  colours,  all  with  different  wavelengths,  in  certain  crystal  structures different colours of light are absorbed  along  different  axes.  All  gems  that  show  pleochroism  are  doubly  refractive,  meaning  they split rays of light within the gemstone. For  example,  if  in  one  direction  all  wavelengths  except for yellow and blue are absorbed, then  the  gem  will  appear  green  (a  mixture  of  the  two colours) from that angle, whereas from a  different direction if all wavelengths but yellow  are  absorbed,  then  it  will  appear  yellow.   As  the two rays have a distinctive mix of colours,  by rotating the gem different colours are seen.  Some pleochroic gems such as Sapphire, Ruby  and Emerald split the light into almost identical  colours and the effect is normally not noticeable.

The level pleochroism is also affected by how  a  gem’s  crystals  are  arranged.  Gems  with  a  single  optic  axis  (tetragonal,  trigonal  and  hexagonal minerals), such  as  Morganite  show  two  colours  or  shades  and  are  sometimes  referred to as dichroic (meaning two colours).  Gems with two optic axes (monoclinic, triclinic  and orthorhombic minerals), such as Tanzanite  and Tourmaline show three colours. The effect  of  which  is  named  trichroic  (meaning  three  colours). It is worth noting that there are never  more than three colours caused by pleochroism.

Due  to  their  crystal  structure,  isometric  gems  including Diamond, Spinel and Garnet do not  demonstrate  any  pleochroism.  When  we  hold  gemstone  courses  at  the  Coloured Rocks Experience,  the  lack of pleochroism in Diamonds often surprises  many people. Its important to remember that the  multiple colours seen in Diamonds as you rotate  them is not due to pleochroism, but is the effect of dispersion.

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Morganite is highly pleochroic.