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
: These minerals fill the pore spaces between sediment grains, acting as a "natural glue" that binds the mass into a hard rock. concretion
Concretions grow from the inside out, typically starting with a nucleus. This process, known as diagenesis, occurs early in the burial history of sediment. The composition of a concretion usually depends on
: A central point—like a leaf, shell, bone, or even a human artifact—attracts dissolved minerals. Ironstone Hematite, Goethite Black Shale High iron content;