It is an abrasive block make from natural stone. Some whetstones are made from manufactured materials such as ceramic, aluminum oxide or carbonium. Whetstones are made with varying degrees of abrasives. The smaller the abrasive material, the finer the stone and the smoother the finish. A whetstone works best when lubricated with a touch of light-grade machine oil or water.
Some stones work properly only when used with water.
The lubricant acts to carry away metal particles as they are removed from the surface of the knife. The lubricant also helps to suspend these particles to prevent them from being ground into the stone's surface. Don't be stingy when using the lubricant. It can make a difference in the finished product. Knife sharpening is a lot like sanding wood where you start with a coarse paper and complete the job with fine paper.