Don’t worry. This isn’t a repeat of Dressgate, nor is it a matter of perception. Iolite, or cordierite, really does change color depending on where the light is coming from. Find out why.
Iolite is the gem-quality version of cordierite. It’s relatively cheap and found all over the place. It can be either a beautiful deep blue or a striking purple color. It should be a good substitute for a sapphire, but it’s not. At least, it isn’t a good substitute without the work of a skilled jeweler.
When a blue chunk of iolite is viewed from the wrong angle, it turns perfectly clear. Turn a purple iolite stone the wrong way and you get an ugly flecked yellow color. Iolite is what’s known as pleochroic. It’s a stone that changes color depending on the angle, and the light, in which it is viewed.
The gem changes color because it polarizes light. As we see it turn in front of a source of polarized light, we can see it get light and dark. Different wavelengths get through its filter as it turns. Some people believe that iolite was the gem that vikings used to figure out the position of the sun on overcast days by identifying the way light from the sky was polarized.
Top Image: Mausweisel
[Source: Cordierite and the Gem Known as Iolite]