A sharpening stone is a tool used to hone and sharpen the edge of a (usually steel) blade or tool.
Sharpening stones are also known as “whet stones”, “oil stones” and “water stones” though these may also be specific types designed for a specific purpose.
These stones come in a wide variety of shapes, sizes, materials and levels of abrasion.
A sharpening stone’s level of abrasion is relative to the size of its grit particles. Coarse stones have large particles and low grit numbers. Fine stones have small particles and high grit numbers.
Because the particles are larger in coarse stones there are fewer of them per square centimetre. In finer stones the particles are much smaller, so there are more of them per square centimetre. A stone’s grit number refers to the number of particles per square centimetre.