#and an amount of voter fraud has me hopeful that certain prompts will be making a definite appearance in the top 26

[just-here-with-my-thoughts]

Iolite is another gemstone which shows eye-visible pleochroism!

It is trichroic, meaning there are three colours in total which can be seen - only two at a time though, remember?

The face-up orientation has been cut to show the combined vivid dark blue and vivid blue wavelengths, producing a pleasing colour.

However as you rotate the stone the vivid dark blue wavelength has to compete with a greyish-yellow near-colourless pleochroic colour! Even without the filter you can see how the colour of the stone washes out when viewed from the side. Using the dichroscope clearly shows the two vastly different colours.

Poorly cut iolite may show the grey-yellow colour through some of the face-up directions, leading to a 'patchy' colour effect.

Fun fact, iolite used to be called 'water sapphire' because of the way the intense blue colour shimmered and disappeared as it turned - like the shifting colour of water!

And did you see I'd updated the original post? Third time lucky trying to upload the video of the tanzanite crystal so you can see the eye-visible pleochroism as I rotate it :)

@royallykt more pleochroism as promised (and @theproblemwithstardust if you're interested?)

Pleochroism in gem quality zoisite var. tanzanite

Apparently we're doing this...

*my pictures, not my crystal

Ever wondered why the colour of some gemstones seems to shift as you view them from different angles? Perhaps are you were looking at a pleochroic gemstone!

Pleochroism is a body colour effect seen in transparent to translucent, optically anisotropic* crystalline material caused by differential absorption of the polarised light rays following different paths through the gemstone, and causing a change to perceived body colour when the stone is viewed from different directions.

...Okay, but what does that mean?

Pleochroism means 'many coloured'. When light passes through a crystal, it is sometimes split into two rays - each ray takes a slightly different path, and because it encounters different atoms within the crystal structure, it is modified to a different colour.

When both different coloured rays of light exit the crystal, your eye resolves them as a single colour. Clever, right? :D

The four photos above are of the same crystal. The top photos show the crystal from two different angles - one looks distinctly more purplish than the other, right? That is eye-visible pleochroism!

Even gems that show pleochroism don't always show it strongly enough to see with the naked eye. But this tanzanite crystal does.

The bottom photos show the same crystals viewed through a type of polarising filter called a dichroscope. The dichroscope is made of polarising film where each half lets through only one of the rays of coloured light - meaning you can see each of the two colours at the same time, one in each half of the filter!

But wait!- I hear you cry. Why are there THREE different colours??

Hmm well this is getting into a deeper discussion of crystal habits and their optical behaviours, so for now let's say that depending on the symmetry of the crystal, an optically anisotropic* stone can show two or three colours - but only ever two in one direction! You have to turn the crystal to see the third colour.

If a stone shows two colours we say it is dichroic, if it shows three it is trichroic. This is the maximum number of colours you will see.

When you view the tanzanite crystal in the position in the left photo, the light rays being transmitted are blue and reddish-purple. Your eyes resolve this to show a purpley-blue colour. Neat!

But in the position from the right photo, it's transmitting blue and yellow. Wild! The yellow ray dilutes some of the intensity of the blue, so the colour you see is a weaker blue tone.

The photo I'm missing from my set is the crystal viewed from top-down, which would then show a purple and yellow split through the dichroscope!

So turning the stone, or moving your head position relative to the position of the stone, really does change its colour, because you are perceiving a different set of coloured light rays being transmitted through the stone!

When fashioning a rough crystal, a lapidary (that's a stone cutter) would to orient the crystal so the best colour is face-up - ie. when you look at the finished polished stone, you will see the most attractive balance of colour. What is considered ideal varies with the stone - you might cut a very dark crystal to show its lighter colour, or a pale one to show the most intense colour it is able to!

Fun additional fact - tanzanite can be heated to improve its colour. It does this by removing the yellow element entirely, meaning you will see intense bluey-purple tones from all angles! The wonders of stone treatments!

*Edit I am misleading you by saying you will see bluey-purple in all directions... in two directions (as tanzanite is a biaxial gemstone) you will find an optic axis, which is a direction of optical isotropy in an otherwise optically anisotropic* gemstone. So in this direction, you will only have a single transmitted ray of light, not two!

*I use the phrase optically anisotropic a couple of times and it's hard to explain without a whole separate citizen lecture on crystallography, but the quick summary is that it means a crystal in which light behaves differently as it travels in different directions - such as the differential absorption resulting in different colours in pleochroism! This is as opposed to an optically isotropic crystal, where light behaves the same way in all directions of travel.

And that is a recited-from-memory summary of pleochroism! :D (please don't tell me off if I got any of it wrong I didn't go upstairs to check my notes ;_;) (I mean I didn't get any of it wrong but my quoted definition probably wasn't word perfect ^^;)

@royallykt thank you so much for your interest i hope you enjoyed learning about all this :)