Bacterial Radar

Achromatium oxaliferum is the only one known bacterium in which calcium (Ca++) is deposited in calcite crystals (crystallized calcium carbonate). No one knows why.

You can see what this special microbe looks like in this nice video:
Achromatium oxaliferum by Michel-Marie Solito de Solis

The Vikings used this material to navigate their ships at sea! In this context the mineral is called Iceland spar , and as a navigation instrument it's called a Sunstone.

You can watch an explanation in this interesting video:
The Sunstone: Secret of Viking Navigation by Terra Incognita

maybe if the crystals in the creature are transparent, but we see the creature as opaque because we see the light entering it on one side and exiting it on another and the light passes two polarizers (crystals) which are not in the same angle, so very little light passes (like two filters).

But to the "eye" of the creature - if it has something like that in its center - each ray of light goes through one polarizer (crystal) only. So the creature can it can see polarized light.

Now the question what is it good for, you can read in Wikipedia in "polarization and vision", which says it's good for navigation.

The creature lives in mud, which means normal visibility is not good. but the mud is made of earth which means minerals, which means crystals. light that goes through crystals become polarized, so this gives the creature a view which is similar to radar!

plus maybe it can identify what's good to eat (identify certain materials which are his food) this way.