sedimentary

Polished septarian nodule

Calcite–aragonite cemented carbonate concretion, commonly with clay minerals and iron oxides

AI-generated identification · may be incorrect

Typically brown to tan with a smooth, polished surface, polygonal shrinkage cracks, and pale mineral-filled fractures; hardness about 3–4 (calcite-rich zones), earthy to waxy luster, and no consistent crystal shape. The rounded form and surface cracking may also resemble a fossilized mud-clast or weathered concretion.

Identified August 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when carbonate-rich sedimentary nodules dry and contract, producing cracks later filled by calcite, aragonite, or barite. Septarian concretions commonly formed during the Cretaceous–Tertiary era, roughly 70–10 million years ago, in marine or coastal mudstones.

Uses & applications

Used as decorative polished stone, display specimens, cabochons, and lapidary material; it is generally unsuitable for construction because of its small size and variable composition.

Geological facts

The distinctive crack network is called septaria, derived from Latin for partition. Acid will usually fizz on exposed calcite veins or freshly scratched surfaces.

Field identification & locations

Commonly found in marine shale and mudstone deposits, especially in Madagascar, Utah, Texas, England, and Morocco. Identify it by its concentric nodule shape and contrasting vein network; definitive identification may require a hardness or acid test.