Septarian is rock not a mineral whose formation is believed to date back to the Cretaceous Period.
This time period in Earth's history began approximately 145 million years ago.
Septarian formed beneath water as minerals and organic matter accumulated around a centre mass.
Over millions of years the mass hardened and became cemented together.
Geologists believe septarian probably formed in shallow bodies of water as the tide caused the accumulated mass to roll back and forth.
With the addition of new layers of mud the size of the mass would have increased.
Although this geological structure is often called septarian, it's correctly known as a septarian nodule or concretion.
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