Walnut shells are the hard outer coverings of walnuts, a type of tree nut. They are typically brown and have a rough texture. Once the nut is harvested, the shell protects the edible part inside until it is cracked open. Walnut shells are often discarded after the nut is consumed. In addition to being a byproduct of nut consumption, walnut shells have various uses. They can be ground into a fine powder for use in abrasives or as a natural filler in products like cosmetics. Some people also use crushed walnut shells in gardening as a natural mulch or soil amendment.