Ghost dust

Ghost dust is a substance used to fight ghosts. First mentioned in Grave Peril and then again in Ghost Story.

Description
Harry Dresden describes the dust as being "'something extra-real, that it was heavy and inert and locked spiritual matter into place when it touched it.'" The dust is able to affect the ghost even in the mortal plane. Upon contact, it sparks into "blazing motes of scarlet light", apparently causing them pain and immobilizing their ectoplasmic flesh.

In the Nevernever, the weight of the ghost dust increases significantly, weighing "thirty or forty pounds" for a small pouch. Harry Dresden believes that, if released, it would tear a hole in the Nevernever.

Though the list of ingredients is incomplete, Dresden mentions a few of them: depleted uranium, cold iron, basil, and dung of some sort. Mortimer uses a different recipe.

Grave Peril
In Grave Peril, Harry Dresden uses it to fight ghosts. He dumps the remaining ghost dust in his pouch onto Leanansidhe, causing her to scream when her skin tear and burn.

Mortimer Lindquist has a variation of it mixed into the paint in his room to prevent ghosts from entering.