Abrasives smooth, finish, grind, or mold workpieces by repeated friction between the abrasive material and the workpiece surface area. Abrasives come in a wide range of shapes and sizes with a choice between natural or synthetic materials. Natural abrasives are comprised of organic minerals like sand, calcium carbonate, and pumice. Synthetic abrasives are made from manufactured materials like zirconia alumina, ceramic, and silicon carbide. Common types of abrasives include flap wheels, honing stones, sanding discs, and finishing discs. Choosing the right abrasive depends on the the desired finish, shape, and cut of the workpiece. Abrasives are assigned a grit size that follows national and international standards for the size of the embedded particles. Abrasives with smaller grit sizes erode surfaces faster, cut deeper, and produce a coarser finish because the particle sizes are larger. Larger grit sizes produce a smoother finish because the abrasive particles are smaller. Aluminum oxide is the most common grain of abrasive material and is used for general-purpose abrading on metal and wood. Zirconia alumina is suitable for more aggressive stock removal on metal and wood.