A sharpening stone is a tool which is designed to help people restore the sharp edges of bladed tools. Sharpening stones for all sorts of tools from knives to axes are available, with varying degrees of hardness and grit which are customized for specific metals. Many hardware stores carry sharpening stones, and kitchen supply stores typically carry sharpening stones which are designed specifically for knives; kitchen stones are also called whetstones.
There are a number of ways to utilize a sharpening stone, but all of them rely on the principle that a blade can be ground into sharpness by being rubbed against a coarse surface. Traditionally, sharpening stones were literally made from stone, although modern sharpening stones may be made from carbon steel, diamond, and a variety of other substances, depending on what they are designed for.
It is possible to break sharpening stones into two basic categories: handheld stones and stones which are mounted on electric grinders. Handheld stones are suitable for fine sharpening work, like knives, while electric grinders can be used for more rugged blades like axes and machetes. Given that different blades require unique sharpening techniques, it should come as no surprise that some people refer to rely on professionals when it comes to sharpening tools, as a bad sharpening job can ruin a tool.
Hand held stones may be designed as blocks which are held in the hand, or as rods which are held by a long handle. The blade to be sharpened is run repeatedly across the face of the sharpening stone to grind out an edge. Depending on how a blade was ground in the first place, it may need to be held at a specific angle or sharpened in a particular way. As the blade is sharpened on one side, a small lip of material called a burr builds up; the burr is removed by flipping the blade and whisking it across the sharpening stone a few times.
Electric stones typically rotate in a circle, requiring the user to hold the blade in question against the grinding face to sharpen it. Using an electric sharpening stone requires skill, as it is easy to grind away a blade or create an uneven grind by not using consistent pressure on the blade. A skilled handler can use an electric grinder for knives, scissors, and an assortment of other tools. Commercial level grinders are capable of sharpening very large, heavy duty blades like those used in paper cutting machines at print shops.