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Rounded concretions are hard, compact masses of mineral matter that form in sedimentary rocks by cementation of mineral-rich pore water. They are typically spherical to ellipsoidal, though some may be irregular or elongated, and they commonly weather out as distinct rounded blocks from surrounding softer rock.

Formation occurs during diagenesis when minerals precipitate from pore fluids around a nucleus, such as a fossil fragment, shell, organic matter, or an mineral grain. Growth often proceeds outward in concentric layers or radially, resulting in a rounded exterior. The surrounding sediment’s chemistry and porosity control the speed and pattern of cementation, producing a range of sizes from centimeters to several decimeters.

Concretions come in various mineral compositions. Common types include calcite or aragonite concretions in carbonate rocks, silica (quartz or chalcedony) concretions in sandstone or shale, and iron oxide or carbonate-oxide nodules in various formations. Septarian concretions, which develop cracks that later fill with calcite or other minerals, are a notable subclass.

Geographic distribution is worldwide, with many famous examples arising in marine sedimentary archives. In some landscapes, rounded concretions weather out prominently and are mistaken for fossils, though they are mineral aggregates rather than skeletal remains. They provide useful information about past fluid chemistry, diagenetic conditions, and the geochemical evolution of sedimentary basins, helping to constrain the history of mineral precipitation and pore-water dynamics.