: A central point—like a leaf, shell, bone, or even a human artifact—attracts dissolved minerals.
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 central point—like a leaf, shell, bone,
: Because concretions are harder than the surrounding rock, they often remain after the softer host rock has weathered away, creating dramatic landscape features. Common Types and Compositions Siliceous Chert, Flint, Jasper Extremely hard and glassy