Hihiirogane (日色金, hihiirogane; Literally "Sun-Colored Metal") is a type of liquid metal that changes its form through the transmission of people's thoughts. The key to refining it, which is the requirement to obtain a swordsmith license, was written down exclusively in the Takeuchi Document.
It is said Hihiirokane glimmers like the sun, never rust and is able to eternally retain its shape, since it is harder than diamond. The Three Sacred Treasures are said to be crafted from the same metal.
Hihiirogane is used as a core in katana to trap the souls in. The Kenshi are able to completely move their soul into the hihiirogane, allowing them to control their katana as if it were their own body. While fulfilling this role, however, the hihiirogane cannot function in any other way. If the hihiirogane is to be used as a weapon, as is the case with Togusa and the Tenka Goken, a second vessel is needed. However, the soul disperses as the vessels increase, so the act of using hihiirogane as a weapon is a difficult task.