concretions are often ovoid or spherical in shape, although irregular shapes also occur.
concretions form within layers of sedimentary strata that have already been deposited.
concretions are usually similar in color to the rock in which they are found.
British Dictionary definitions for concretions
the act or process of coming or growing together; coalescence
a solid or solidified mass
something made real, tangible, or specific
any of various rounded or irregular mineral masses formed by chemical precipitation around a nucleus, such as a bone or shell, that is different in composition from the sedimentary rock that surrounds it