Moeraki Boulders on the East Coast of South Island. These boulders are called concretions, and were formed in ancient sea floor sediments. They were created by a process similar to the formation of pearls, where layers of material cover a central nucleus. For the oyster, this nucleus is a grain of sand. For the boulders, it was a fossil shell, fragment of bone, or a piece of wood.
Moeraki Boulders, South Island, NZ