Instead, it's a rock formation that happens when minerals precipitate out of groundwater and cement surrounding sediment. This process can occur within various types of rocks, including:

* Sedimentary rocks: These are the most common locations for concretions, as they often form within layers of sand, clay, or other sediments.

* Igneous rocks: Concretions can also form within igneous rocks, particularly in areas where groundwater has infiltrated the rock and reacted with minerals.

* Metamorphic rocks: While less common, concretions can develop in metamorphic rocks as well.

Therefore, to determine the type of rock a concretion is part of, you need to consider the surrounding rock it's found in.