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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.
Morganite is highly pleochroic.
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