Iolit / cordierite is a mineral from the group of cyclosilicates rich in aluminum. Its crystals show pleochroism - from one point of view it is deep blue-violet, when the stone turns blue disappears completely and appears to be gray to yellowish smoke. The third color is less noticeable, mostly pale blue.
To a greater or lesser extent, the pleochroic majority of precious stones except those that crystallize in the cubic system. For most of them, pleochroism is not very noticeable, and a simple optical device, called dichroscope, is used to reveal it. With prominent, strong pleochroism, it can be found especially in some sapphires, tourmaline and non-burned tanzanite, but rarely as noticeable as in cordierite.
The color direction is given by the orientation of the crystal axis. Stones that have 2 colors are called dichroic, with 3 trichroic colors. At times when this phenomenon was not yet explored in detail, the cordierite was called dichroit, although it is actually trichroic. It is worth mentioning that it is also found in the Czech Republic (for example, in Šumava).
Reference: IO 10926