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Though the processes by which concretions form are poorly characterized, it is believed they form during the diagenesis of a deposit, usually shortly after the enclosing sediment has been buried. They are believed to occur when a considerable amount of cementing material precipitates locally around a nucleus, often organic, such as a leaf, tooth, piece of shell or fossil.

Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several hundred pounds. The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost 10 feet in diameter. Concretions are usually similar in color to the rock in which they are found. They are commonly composed of a carbonate mineral such as calcite; an amorphous or microcrystalline form of silica such as chert, flint, or jasper; or sometimes an iron oxide or hydroxide such as goethite. They can also be composed of other sedimentary minerals that include dolomiteFor the European mountain range, see Dolomites Dolomite is a mineral (formula CaMg(CO)) consisting of a calcium magnesium carbonate found in crystals and in beds as dolostone. A pure form of dolostone would be rare, however; it usually intergrades with li, ankeriteAnkerite an iron-rich dolomite often associated with gold deposits., sideriteSiderite is a mineral composed of iron carbonate FeCO. It is an valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Both magnesium and manganese commonly substitute for the iron. Its crystals belong to the hexagonal system, a, pyriteThe mineral pyrite or iron pyrite is iron disulfide, FeS. It has isometric crystals that usually appear as cubes or pyritohedrons. It has a slightly uneven and conchoidal fracture, a hardness of 6-6. 5, and a specific gravity of 4. Its metallic luster and, bariteBarite ( Ba S O) is a mineral consisting of barium sulfate . It is generally white or colorless, and is the main source of barium. Baryte is the British spelling. Barite commonly occurs in lead-zinc veins in limestones, in hot spring deposits, and with he and gypsumGypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO 2HO. Chemical structure Heating gypsum above approximately 150°C (302°F) partially dehydrates the mineral, by driving off exactly 75% of the water containe, to name a few.

Concretions are found in a wide variety of rocks, and are particularly common in shaleShale is a fine-grained sedimentary rock whose original constituents were clays or muds. It is characterized by thin laminae breaking with an irregular curving fracture, often splintery, and parallel to the often indistinguishable bedding planes. Non-fisss, siltstoneSiltstone is a sedimentary rock that is intermediate in grain size between the coarser sandstone and the finer shale. As its name implies, it is primarily composed of silt, defined as grains smaller than 62 micrometres. Siltstones were at one time often rs, and sandstones. They often outwardly resemble fossils or rocks that look as if they do not belong to the strata in which they were found. Occasionally, concretions contain a fossil either as its nucleus or as a component that was incorporated during its growth, but concretions are not fossils themselves. They appear in nodular patches, concentrated along bedding planes, protruding from weathered cliffsides, randomly distributed over mudhills or perched on soft pedestals.

Septarian concretions (or Septarian nodules) are terms used to describe concretions containing angular cavities apparently caused by differential shrinkage of their interiors. The cavities usually contain crystals precipitated from circulating solutions, usually of the mineral calcite.

Descriptions dating from the 18th century attest to the fact that concretions have long been regarded as fascinating geologic curiosities. Because of the variety of unusual shapes, sizes and compositions, concretions have been variously interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extra-terrestrial debris or human artifacts. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens.

The word "concretion" is derived from the Latin "con"-- meaning "together" -- and "cresco" -- meaning "to grow."