A concretion is a hard, compact mass of mineral matter that forms within sedimentary rock or soil. While often mistaken for dinosaur eggs, fossils, or meteorites, they are distinct geological features formed by the selective precipitation of minerals from groundwater. How Concretions Form
The composition of a concretion usually depends on the mineral present as a minor component of the host rock. Main Mineral Key Feature Calcite, Siderite Sandstone, Shale Often spherical; most common type. Siliceous Chert, Flint, Jasper Extremely hard and glassy texture. Ironstone Hematite, Goethite Black Shale High iron content; can look like rusted metal. Septarian Calcite, Siderite Clay, Mudstone Features internal radial cracks or "septaria". Notable Examples concretion
: These minerals fill the pore spaces between sediment grains, acting as a "natural glue" that binds the mass into a hard rock. A concretion is a hard, compact mass of