Pleochroism

Pleochroism

Pleochroism is probably one of the coolest optical phenomena that minerals exhibit. The word pleochroism means “many colours” and comes from the Greek words “pleo” meaning more and “kroma” meaning colour. It is essentially when a mineral shows different colours or different intensities of colour based on the angle you view it from. This phenomenon can be seen in a variety of popular minerals such as tourmaline, tanzanite and emerald which are often cut into gemstones and worn in jewellery.

Why does Pleochroism occur?

Pleochroism occurs due to the crystalline structure of a mineral. The crystal structure determines how the different wavelengths of light are absorbed and refracted, thus affecting how we perceive the colours.

a> Isotropic Crystals

Pleochroism is never seen in minerals that are part of the isotropic crystal system. These crystals have a uniform refractive index throughout the stone. Uniform refractive index means light rays behave the same in all directions of propagation and therefore show a uniform colour no matter which angle the mineral specimen is viewed from. Examples of isotropic crystals are pyrite and diamond.

Note: The isotropic crystal system is also called cubic crystal system

b> Anisotropic Crystals

All minerals from the other six crystal systems show pleochroism because they are all anisotropic in nature. This is because anisotropic crystals have the ability to doubly reflect light.

Uniaxial crystal systems (tetragonal, trigonal and hexagonal) possess a single optical axis thereby enabling them to exhibit dichroism, which means that only two colours will be visible. Biaxial crystal systems (monoclinic, triclinic and orthorhombic) exhibit trichroism, a result of three colours being visible.

All coloured anisotropic crystals are pleochroic but many don’t have distinctly visible pleochroism.

Uses of Pleochroism

Pleochroism can be an extremely useful tool to identify minerals and gems since similar looking minerals and gemstones can have very different pleochroic colour schemes. They can also make for an extremely pleasing viewing experience since seeing a stone seemingly change colour simply by viewing it from a different angle can resemble a magic trick.

How to detect Pleochroism

In certain specimens of gemstones or minerals, their pleochroic properties can be seen with the naked eye. However, lots of gemstones which are pleochroic possess a weak pleochroism which could be hard or even impossible to notice with your eyes alone. In other gemstones, the material might be too thick and might not let enough light through for proper observation, which makes it difficult to determine whether or not it is pleochroic.

a> Dichroscope

In stones where it’s hard to detect pleochroism with the naked eye, we can use a special device called a dichroscope. This instrument separates two polarizing light beams so that they can be viewed individually. When you look through a dichroscope you will see two small windows of colour. These windows are created by the optical calcite (or a polarizing filter) present in the dichroscope. The colours of the two windows could be identical to each other or they could appear different. Carefully observe the colours of the two windows (this should be easy as they are placed side by side) and see if you notice a difference in colour between them even if it’s very subtle. If there is a difference in colour we can conclude that the gem is at the very least dichroic. If there isn’t any difference in colour the gem could be isotropic or you could be viewing it from a direction that doesn’t show a difference in colour. That is why it is important to reorient the dichroscope in several other directions and observe the gem to see whether or not it’s truly isotropic because you will not be able to see the dichroism of a biaxial stone if you are viewing it from its optical parallel axes.

When it comes to trichroic gems, start by observing one pair of colours and then shift the gem with respect to the dichroscope to view a second pair of colours (one of the colours will be common between both the first and second pair).

b> Petrographic Microscope

Another method is to use polarized light microscopy in which a petrographic microscope is used to examine a thin section of a mineral under polarised transmitted light.

Degrees of Pleochroism

Not all minerals exhibit the same intensity of pleochroism. This is why we categorise pleochroic quality into three groups - strong, moderate and weak.

Conclusion

This phenomenon occurs in a plethora of stones, ranging in price and rarity. In my opinion, every avid collector of gemstones or minerals should possess at least one specimen showing pleochroic ability.

The stone that inspired me to write this blog was a gorgeous piece of tanzanite that I got my hands on. Tanzanites are strongly pleochroic and their pleochroism can be viewed without a dichroscope. You simply need to turn the gem around in different directions to see the different colours.

Since tanzanite is trichroic, it exhibits 3 colours - red/violet, deep blue and a greenish yellow.

I have added a couple of videos of the tanzanite displaying pleochroic behaviour.