Pleochroism – What is it?

Pleochroism is visual property that only occurs in double refractive gemstones. The are some gemstones like Tourmaline, Tanzanite and Ioloite that have very obvious pleochroism. But what is it? A picture says a thousand words so lets have a look at an example of Tanzanite.

 

This is the same piece of Tanzanite looking through different polarized filters. It shows the colours that Tanzanite is capable of showing.

Pleochroism is the ability of a gemstone to show different colours depending on which angle it is viewed. Lets look at a piece of Iolite. Iolite is trichroic, meaning it displays three distinct colours.  These colours are blue-violet, clear and yellow. Lets have a look at a piece of Iolite and see if the three colours can be seen clearly.

The square piece above clearly shows the different colours that Iolite can display when looking at it in different direction.

Tanzanite is another gemstone whose pleochroic colours can been seen easily. Usually it is easy to see the violet and blue colours

 

Imagine for a minute that the above Tanzanite was cut into a gemstone. If the table of the stone (the top) was in the same direction as the Tanzanite crystal on the left, the Tanzanite would look blue to our eyes. If however the Tanzanite was cut in the direction of the same crystal shown in a different direction on the right, we would see a purple Tanzanite.

It is easy to see this with a cut Tanzanite. In the picture below, the table of the Tanzanite is blue, whereas the edges on the gem are slightly purple. This is because the cut of the gemstone allow us to see the other pleochroic colours in Tanzanite.

Iolite, as we saw before, is strongly pleochroic. Lets have a look at a cut stone and see what effect it can have on the cut gemstone

 

Tourmaline is one of my favourite pleochroic gemstones. It is usually very easy to see the different colours of this gemstone just by holding it up and rotating it. This is one of my favourite pieces that shows the effect of pleochroism.

 

Look at some of your gemstones and see if you can see any effects of pleochroism. Hold them in your fingers and rotate them in all different direction and enjoy the different colours your gem can produce that you didn’t even know about!