A good septarian "egg",

Septarians were formed during the Cretaceous period about 65 million years ago. Decomposing sealife killed by volcanic eruptions had a chemical attraction for the sediment around them, forming mud balls. As the ocean receded, the balls were left to dry and crack. Decomposing calcite from sea-shells was carried down into the cracks in the balls forming calcite crystals. These are the dark brown crystals lining the cavity - here they are Aragonite, a form of calcite. The name is derived from the Latin septem, meaning seven, as the mud balls crack with seven points in every direction creating the beautiful design