Concretions

Concretions:

1. During diagenesis certain constituents of sediments (or new chemical precipitates from pore waters) tend to concentrate in certain parts of the rock, often accumulating around a nucleus. The masses so formed may be rounded or irregular, and are known as concretions. Concretions composed of clay ironstone, limestone, dolomite, chert (flint), gypsum, barytes, and limonite have been recorded.
2. A term applied to primary aggregates developed during sedimentation, which form discrete masses enclosed in the main body of sediment. Manganese oxide concretions are forming today in the abyssal regions, while chert, calcium phosphate, and calcareous types are also known from modern and ancient sediments. Some early concretions are related.