The Prokaryote Stone is a hard, smooth, brown-colored object that, according to Giles, can move within a person's mind to unleash "ideas, images, memories" that he or she might have forgotten or suppressed. Giles used it to gather information about the psychological trigger the First Evil was using to control Spike. He enlisted Willow to recite a spell that transformed the Prokaryote Stone into a silvery, worm-like creature that crawled past Spike's right eyeball and then invaded his brain through his optic nerve. It induced a vivid recollection where Spike heard the song that was the trigger. When the Prokaryote Stone left his body, Spike remembered the song. In this way Giles and the others learned what the trigger was, but not how to free Spike of its influence. Spike later remembered the experiences of siring and then staking his mother.
The word "prokaryote" means a cellular organism that has no nuclear membrane and no organelles in the cytoplasm except ribosomes. Its genetic material is in the form of single continuous strands. Prokaryotes include bacteria and blue-green algae and make up the kingdom Monera.