The Shigyoku is mineralized spirit matter and usually takes the form of a small bead or jewel. It can be any shape, size, and color, and usually reflects the nature of the spirit's aura.
The Shigyoku serves several functions. It serves as a reservoir of power for a downed Angel. It also serves as the channeling gate to the Physical Realm after they have been cast into Hell.
Scientists working on Project ESPer discovered that samples could be taken from the Shigyoku and used to recreate some of the Celestial's will and presence. Using the Shigyoku and blank souls, Ochigami were able to be recreated from Shigyoku remnants. This led to the creation of the Vespers.