Patterns when you rub/screw up your eyes

If I close my eyes and rub them, or even just screw them up tightly, I see geometric patterns, usually concentric dark and light diamonds or squares, often “cycling” in and out from the centre, with clear, sharp corners.

Does everyone see these, and does anyone know what causes them? Pressure on the optic nerve, perhaps, but why angular shapes?

It’s a well-known and common phenomenon. The geometric patterns you see are called phosphenes.

Wikipedia article on them.

Wikipedia has everything. I had heard of “stars” as phosphenes, but didn’t know this was the same thing. The patterns I see aren’t quite like the artist’s impression, though. The “grids” are much larger. And it seems nobody really knows why the brain interprets the signals as geometric patterns rather than random blobs.