Gallery of Concretions

I asked my newsletter subscribers for pictures of concretions, and now there are enough to fill this new gallery of concretions. These intriguing objects are often taken for fossils, but they form purely from mineral deposition in newly buried sediment that is just beginning to become rock—that is, they arise during early diagenesis. Millions of years later, they erode out of their matrix and in some places they make impressive spectacles.

Bowling Ball Beach — Courtesy Chris de Rham under Creative Commons license