a very hard block, usually made of a mixture of silicone and carbon called carborundum
Extremely hard, fine grained blocks of carborundum that are commercially sold to sharpen knives and cutting tools. Knives should be periodically honed on whetstones to keep them sharp.
A whetstone is a stone used for sharpening the edges of tools.
Fine grindstone that is used to sharpen the edge of a tool, manually.