Sulfur, or sulphur, is a non-metallic element that is found normally on the earth's crust, areas with volcanic activities, and is known in the Bible as brimstone. The most notable use of sulfur in the supernatural world is that it is a key marker of demonic manifestation. Since it is an identifier of demonic activity, it is thus useful for hunters when identifying creatures responsible for supernatural cases. It appears that the more powerful the demon, the more sulfur they leave behind. Most demons merely leave traces of the stuff, while Azazel left piles of it.
Sulfur is found in powder form wherever demons manifest. The smell of rotten eggs is given off by sulfur. Because of this, part of the rhetoric of hunters is to ask victims if they caught the scent of rotten eggs.
Since that is a type of dust, if someone pours that in front of a fairy, probably they have to stop to count each grain one by one.