sedimentary

Polished septarian nodule

Septarian concretion, commonly calcite-filled cracks in a claystone or limestone matrix

AI-generated identification · may be incorrect

Dark gray to black, rounded and polished, with tan-golden vein fillings; matrix is usually calcite-rich claystone, with Mohs hardness about 3–4, calcite veins 3, earthy to waxy luster, and no consistent crystal habit visible.

Identified August 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when a mud-rich concretion dries and contracts, producing radial cracks later filled by calcite, aragonite, or occasionally barite. Most attractive examples formed in marine sediments during the Cretaceous–Eocene, though age varies by locality.

Uses & applications

Used as a decorative polished stone, cabochon, paperweight, and collector specimen; rarely used in construction because it is ornamental and relatively soft.

Geological facts

The contrasting vein network is caused by mineral precipitation after the original concretion formed. Golden-brown veins may be calcite, aragonite, or iron-stained carbonate minerals.

Field identification & locations

Identify by its rounded concretionary form, dark mudstone matrix, and angular cream-to-gold crack pattern; common sources include Utah and Madagascar. Acid may fizz on exposed calcite, but testing can damage the polish.