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In the past, Hibiki's body was fused with the fragments of Gungnir (the first medical case of a fusion between human and sacrist). As the fusion grew deeper, it gave birth to various supernatural phenomena. One example of this was Spontaneous Combustion - the abnormal rise of Hibiki's body temperature to extreme levels. The inside of her body became like a kiln, creating a substance that eventually grew outside of the body and was collected. The research teams determined the substance had a composition extremely similar to that of the human body and the phase during which it was created was fixed at an imaginary number. Various parts of it differed from the makeup of the human body and it was temporarily filed as an undiscovered substance. Up until this point, it had been stored at a special location along with other unanalyzed objects that are controlled by the Japanese Government in cooperation with the United Nations. When Elfnein read the report about this object she didn't approach it scientifically as the research teams did, but rather applied her own unique approach as an Alchemist. She realized that the identity of the substance was like a "Philosopher's stone set in a negative phase". From there, she began to build her hypothesis.
In AXZ Episode 8, Elfnein used her alchemy to integrate it into the Symphogear System it became possible for the Symphogear users to once again use the Ignite Module. It acts as a "Nullification Barrier Coating" that prevents the Philosopher's Stone from forcibly canceling the Ignite form, and brings their combat potential back on an even level to the Faust Robe.
- As the Fool's Stone is intentionally named by Chris, Hibiki didn't even like it. The origin and nature behind the stone's name is a reference to Hibiki's high level of stress that cause the fragment of Gungnir to grow more, which puts her into a stubborn situation while getting into a serious risk.