Birefringent. Not that there’s anything wrong with that.
In my earlier discussion of polarization, I promised some photos of stress-induced birefringence.
If you have a polarizing filter, you can use an LCD as a polarized light source and view birefringent materials using it as a backdrop. Make the screen as white as possible, and rotate the polarizer until it blocks the light. Then place a birefringent material in front of the screen and look through the polarizer. Cheap clear plastic often will have stress-induced birefringence.
Which is what I did, and is why the background is black. The first photo is the plastic box in which the polarizers were packaged
These next two are the side and top views of a styrene drawer from a small storage cabinet, placed on top of an empty CD spindle.