Moeraki Boulder

The Moeraki Boulders are unusually large spherical boulders lying along a stretch of Koekohe Beach on the wave-cut Otago coast of New Zealand between Moeraki and Hampden. They occur scattered either as isolated or clusters of boulders within a stretch of beach where they have been protected in a scientific reserve. These boulders are grey-colored septarian concretions, which have been exhumed from the mudstone and bedrock enclosing them and concentrated on the beach by coastal erosion. Especially in recent years, the boulders have been a popular tourist attraction.

Source: Wikipedia