"Cannonball (can'-non-ball) A large, dark concretion, as much as 3 m in diameter,
resembling a cannonball, as in the Cannonball Member (Paleocene) of the Fort Union Formation in the Dakotas. " and "concretion (con-cre'-tion) (a) A hard, compact mass or aggregate of mineral matter, normally subspherical but commonly oblate, disk-shaped, or irregular with odd or fantastic outlines; formed by precipitation from aqueous solution about a nucleus or center, such as a leaf, shell, bone, or fossil, in the pores of a sedimentary or fragmental volcanic rock, and usually of a composition widely different from that of the rock in which it is found and from which it is rather sharply separated. It represents a concentration of some minor constituent of the enclosing rock or of cementing material, such as silica (chert), calcite, dolomite, iron
oxide, pyrite, or gypsum, and it ranges in size from a small pellet-like object to a great
spheroidal body as much as 3 m in diameter. Most concretions were formed during
diagenesis, and many (especially in limestone and shale) shortly after sediment deposition."
Glossary of Geology 5th Edition