A carborundum stone is a sharpening stone made from silicon carbide abrasive fused into a hard block, used for grinding and honing knife edges. Harder and more aggressive than natural whetstones, it removes metal quickly, making it suitable for restoring very dull or damaged blades, though it wears the edge faster than finer stones and is often followed by a smoother stone for finishing. Some are shaped like a butcher’s steel for the same stropping action. Regular flattening keeps the surface true. For kitchen knives, it is a workshop-grade tool: effective but unforgiving, best used with water as a lubricant and a steady angle.