Diagenetic concretions are masses of minerals formed by diagenesis (the process of sediment turning into sedimentary rock), and can be formed in situ or weathered out. They vary in morphology: they may have visible sedimentary laminations (fine discrete layers), and they can be spherical, discoidal, or elongate in shape. They do not displace sedimentary layers. Their interiors may be solid, rinded, or layered.
Spherical concretions can form in self-organized (non-random) or random spacing within the three dimensions of the host rock, and their spatial distributions are linked to how the spheroids formed.
Clusters of concretions (elongate along sedimentary laminae) can be biotic when nucleated on organics concentrated on bedding planes. Also, clusters of concretions are observed on organic nuclei such as preserved bones. Irregular shapes can also preserve biotic nuclei, such as entombing shells or other fossils.