DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Septarian nodule. This is a round mineral deposit of claystone (grey) filled with cracks formed when the claystone dried up. These cracks fill with other minerals, often calcite, a form of calcium carbonate. The variety of patterns that can form are often prized for their beauty. This specimen, from the Appenine mountains, Italy, measures 450 millimetres in diameter.
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