Cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate.
Concretions are usually similar in color to the rock in which they are found.
Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-like aggregates.
This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.
There is an important distinction to draw between concretions and nodules.
Concretions form within layers of sedimentary strata that have already been deposited.
They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock.Because of the variety of unusual shapes, sizes and compositions, concretions have been interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extraterrestrial debris or human artifacts.Detailed studies (i.e., Boles et al., 1985; Thyne and Boles, 1989; Scotchman, 1991; Mozley and Burns, 1993; Mc Bride et al., 2003; Chan et al., 2005; Mozley and Davis, 2005) published in peer-reviewed journals have demonstrated that concretions form after sediments are buried but before the sediment is fully lithified during diagenesis.Although concretions often consist of a single dominant mineral, other minerals can be present depending on the environmental conditions which created them.For example, carbonate concretions, which form in response to the reduction of sulfates by bacteria, often contain minor percentages of pyrite.Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.