Concretions are hard bodies that form in sediments before they become sedimentary rocks. Slow chemical changes, perhaps related to microbial activity, cause minerals to come out of the groundwater and cement the sediment together. Most often the cementing mineral is calcite, but the brown, iron-bearing carbonate mineral siderite is also common. Some concretions have a central particle, such as a fossil, that triggered the cementation. Others have a void, perhaps where a central object dissolved away, and others have nothing special inside, perhaps because the cementation was imposed from outside.
A concretion consists of the same material as the rock around it, plus the cementing mineral, whereas a nodule (like flint nodules in limestone) is composed of different material.
Concretions can be shaped like cylinders, sheets, nearly perfect spheres and everything in between. In size they can range from as small as gravel to as large as a truck. This gallery goes from small to large.