These are very hard, high in iron and can be as large as volkswagens. Tiny iron concretion particles cause specking in fired ware, especially in reduction.
Specking is a fault in fired ceramic ware where speckles of ion bearing contaminants are visible and mar the appearance of the surface.
A method of firing stoneware where the kiln air intakes and burners are set to restrict or eliminate oxygen in the kiln such that metallic oxides convert to their reduced metallic state.
Ironstone at wikipedia