Hihi'irokane (ヒヒイロカネ or 緋々色金, lit. crimson ore) is a red-colored mythic metal or alloy depicted in some ancient Japanese legends that allegedly possessed strange physical properties, and could be used to forge some extraordinary metal alloys. It is analogous to other fictional or mythic metals such as Orichalcum or Mithril.
Described in the apocryphal Takenouchi Document, an alleged ancient writing in a lost script which details Japan's early history, Hihi'irokane was used in the time of Emperor Jimmu, Japan's first emperor. The Sword of Kusanagi and the other Imperial Regalia of Japan are supposedly made from it.
Its weight is lighter than gold, but harder than diamond. It does not rust. It was even said to be able to bring water to a boil without heat, violating the Law of Conservation of Energy.
In Touhou Project
In Touhou, it is a rare metal. After hearing Rinnosuke Morichika and her father argue about it when she was younger, Marisa became obsessed with finding some for herself and takes to collecting metal scrap. Years later, after her fruitless efforts, she approaches Rinnosuke and asks him to repair her Mini-Hakkero with it, since it had gotten rusty. Though he denies having any, he relents under questioning and agrees to the repairs using what little he had.
However, since Rinnosuke surmises her metal collection had merely become junk to her and that she wouldn't really know Hihi'irokane when she saw, he talks her into a trade for the scrap. In it, he finds the legendary Kusanagi, forged from the metal which Marisa had been seeking.