The Belgian stone, also called the Belgian Old Rock, used to be a superb natural hone for razor sharpening. It is used dry, with water or with oil, and has been the standard razor hone for centuries. It is still quarried in Belgium, in the caves around Liege, but due to some earthquakes the production process has been delayed. However, the best stones are already gone, and it is getting increasingly difficult to acquire modern ones with the same quality as those from the previous century. Therefore, due to irregularities, all present natural stones carry a considerable risk of damaging the edge of the razor.
Synthetic stones contain bonded abrasive particles.
They are either completely synthetic or reconstituted from crushed natural stones. Some are to be used with water, some with oil, but all natural and synthetic stones might also be used dry. Examples of man made oil stones are silicon carbide and aluminium oxide stones with vitrified bonds. Woodworkers use the coarser ones to do some quick grinding, but for fine honing there are better alternatives. Oil will relatively soon mask the abrasive particles in a synthetic oil stone because of the type of bond.