A Ghoststone is a stone used to create fake ghosts in a factory. It is said these stones have existed for thousands of years and absorb the evil in humans, in order to create ghosts.
In "Les Diaboliques", Scanty and Kneesocks were using a Ghoststone in their factory below Daten City High to create a huge army of fake ghosts. However, the Anarchy sisters' destroyed it, resulting in the entire factory and the school being destroyed.