Naniwa Diamond Stones are aluminium 'stones' covered with a layer of about 2mm of water stone, with diamond as an abrasive material. This makes them genuine Japanese water stones and, unlike other diamond stones, they last for a very long time! The diamond powder also ensures that you can hone your equipment very rapidly - in half the time it would usually take with normal water stones. These stones are ideal for use with knives made from the hardest form of steel.