What are the geometrical prisms that sparkle in my vision every so often only in my left eye?

Migraine: Mostly likely this is a visual or occipital migraine. If you cover each eye in turn when this happens you will see that this is in 1/2 of the left side of the vision from either eye. These usually last 5-60 miinutes, have no aura and rarely an associated headaches. They are benign, have no identifiable cause and don't leave any damage when they fade out.

Likely migraine: You likely have retinal/ocular migraines which have visual phenomena with or without headache. It's still necessary to have a dilated retinal exam to rule out a more severe reason.