The pieces you’re probably referring to are the inside core stem of the mugwort! When we process the mugwort, the thicker stems break up and get sifted out, but a little bit of what’s on the inside of the stem (porous plant matter) gets mixed into the batch. It’s just not big enough for the sifter to catch. The pieces are not styrofoam, or vermiculite.  They are just part of the mugwort plant.  It is harmless to smoke and is part of the herbs, even though it looks funny!