Large spherical boulders -- some measuring 12 feet in circumference -- are scattered on Koekohe Beach on the east coast of New Zealand's South Island. They formed millions of years ago on the ancient sea floor, collecting and hardening sediment and minerals around a core such as a fossil or a shell similar to the way oysters form pearls.
They're not the world's only examples of what geologists call septarian concretions. You can also visit the Koutu Boulders near Hokianga Harbour on the northwestern coast of New Zealand's North Island, for example. Yet the Moeraki Boulders are some of the world's largest. The particulars of their origin and what caused the distinctive cracks inside them are still being studied.
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