Natural abrasives (garnet and flint) go from 60 (coarse) to 220 or 260 (fine) grit size, which is generally enough range for finishing wooden projects. With synthetic abrasives such as silicon carbide and aluminum oxide grit sizes go up and over 1500, which are extremely fine grits for delicate materials such as metal or plastic. It is hard to answer the question of what is the highest grit sandpaper, as the technology develops further and these numbers can go up even higher.