Artificial stone and substitute for Ara-to. The first stone of the groundwork, used before Binsui-do. Kongo-do is used when the sword is rusted or has a flaw.  Asahi Kongo-do No. 180 Hard Type is good for hard swords such as gendai-to. It should not be used heavily as it takes away a lot of metal. Dimensions 20.5 x 5 x 7.5 cm,  weight 1.4 kg.