Scanning electron micrographs demonstrate the presence of microcrystalline structures on the surface of coal-derived fly ash samples taken from electrostatic precipitator hoppers. Cenospheres (hollow spheres) were found to be packed with smaller cenospheres, which were also packed with spheres. Microspheres, apparently formed by uneven heating, are encapsulated in the parent sphere. Chemical analyses provide a basis for the postulation of a mechanism of formation for plerospheres (hollow spheres packed with spheres) and microcrystals. (Ed. note: See also accession number 19129).