The wheels, metal grinding stones (emery wheels) consist of hard grains either of aluminum oxide (aluminum oxide) or silicon carbide (silicon carbide). The brown and gray wheels that are general purpose (twin wheels drills etc.) consist of 95% and 97% aluminum oxide respectively. The white ones, which are for harder materials such as HSS, INOX, consist of 99% aluminum oxide. Finally, the green ones are for carbides or marble and consist of 99% silicon carbide.