Ear rocks are small crystals of calcium carbonate called "otoconia" which collect in the inner ear.
Benign Paroxysmal Positional Vertigo (BPPV), BPPV, a form of vertigo, is thought to cause dizziness from debris that builds up in the inner ear (utricle). This debris is referred to by some as “ear rocks” or otoconia.
Inner Ear 'Rock Slides' Lead To Vertigo : NPR
As the head changes position during treatment, the ear rocks continuously move to the lowest point