Sweater Stone is a natural pumice which has the unique ability to remove pilling from knits with each sharp edge of a broken cell. It actually cuts the filament that holds the pill to the garment while the cell breaks off, removing pilling and restoring the nap on fabrics.
Pilling commonly occurs on a fabric when fibers that are worked loose by rubbing, form little balls. The pills don't fall off because they are held to the fabric by the fibers that still hold firm. Loosely woven and knitted synthetics and blends are the typical candidates for pilling. Usually, it's the stronger synthetic fibers that hold the pill to the surface. Pilling can happen to clothes, blankets, and upholstery fabrics. However, sweaters seem especially vulnerable.