Rock tumbling simulates the natural polishing action of the ocean's tide, which rolls rocks and shells (and even glass!) around in the sand and water. This grinding process continues for many, many years and is why most of the stones you find at the beach (or in a stream, for that matter) are so smooth. A rotating tumbler mixes rocks, water, and carefully-graded abrasive materials to achieve the same (or better) results in far less time.

Starting with a very coarse sand-like material, and moving through three more stages of ever-finer grit, it will take your tumbler only four to six weeks to accomplish what it takes Mother Earth years (if not centuries) to do. To really understand how tumbling works, just think about how you would go about sanding a flat surface. You would start with a very course sandpaper and progress to finer and finer sandpaper until the surface was very smooth. Tumbling simply adds water (and a third dimension!) to the process.