The stone used in a knife sharpener is composed of silicon carbide and is usually a rectangular shape with a thickness of about 1/2 inch to several inches. Its two main types are whetstone and carborundum stone. The first is made of silicone dioxide minerals, while the second is of carbon. The stones are typically classified by hardness, with the hardest stones removing the most material. The stone is then oiled with a light weight lubricating oil.
Unlike ceramic and diamond materials, steels used for sharpening knives have different functions. The first is to restore the edge of the blade by removing any excess material. The second type is called a honing steel, which does not sharpen the blade, but is used to restore a bent edge. The honing steels are very difficult to clean and should only be used for knives that are very dull.
Another type of stone used in sharpening knives is diamond. It is used to straighten out a bent knife blade. It is usually sold as a set of two stones. The diamond whetstone is made from a perforated steel sheet and is mounted in a natural wood box. It is covered with tiny diamond particles that are embedded in an electrodeposited nickel. Hence, it is called a honing steel.