Variously colored, translucent spheres in the Compilation of Final Fantasy VII. In-story, condensed mako energy, derived from the Lifestream, granting powers based on its type. The white materia is smaller than normal, about marble-sized, while the black materia is larger than most fists.
These and the equipment slots for their insertion can be seen in FFVII designs. Often associated with Yuffie Kisaragi.