Eggshell / Cascarilla

It helps block non-corporal beings from a physical space so pressing it into chalk form, or sprinkling it can work as a barrier to keep unwanted nasties at bay.  
Some say it even blocks out the benign energies at work as well, so it’s great for those times when you need to just sit and be a muggle for a little while. 
Sprinkle it across thresholds such as windows and doors or even use it to draw sigils. 
It can also be used to enhance psychic ability in a way because it protects you from that which seeks to take advantage of you when your engaged in psychic activity. 
Simply rubbing a little of the powder on you or dabbing a dot onto your third eye energy point is said achieve this. 
Another use is purification and is especially useful in the bath. 
It dissolves easily in water so adding it to a bath intended for purification can amp up the cleansing effect.