Charcoal

Activated Charcoal is a black charcoal powder made from nut shells, coconut shells, coal, peat or wood products.

Can be found at health food stores in capsule form and at soap making supply stores. 

Said to act like a magnet carrying toxins from the body.

May neutralize odours which can make scenting the soap more difficult.

Source : http://www.soap-making-essentials.com/mineral-soap-ingredients.html