sedimentary

Septarian nodule (likely polished)

Calcite- and/or aragonite-filled mudstone concretion; commonly Fe-bearing claystone/mudstone matrix

AI-generated identification · may be incorrect

Brown rounded concretion with cream-colored polygonal veins. Matrix hardness is roughly 3–5 Mohs; calcite veins are 3, with earthy to dull luster and no obvious crystal faces.

Identified August 22, 2026More sedimentary →

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Physical properties

Brown rounded concretion with cream-colored polygonal veins. Matrix hardness is roughly 3–5 Mohs; calcite veins are 3, with earthy to dull luster and no obvious crystal faces.

Formation & geological history

Forms when shrinkage cracks develop in a carbonate-rich mud concretion during burial, then become filled by calcite or aragonite and sometimes barite. Many attractive examples formed in Cretaceous–Tertiary marine sediments, commonly tens of millions of years old.

Uses & applications

Used as a lapidary, display, and metaphysical specimen; cut and polished pieces are popular decorative stones. It has little industrial value.

Geological facts

“Septarian” refers to the internal crack network, from Latin septum, meaning partition. The brown color commonly reflects iron oxides or organic-rich sediment.

Field identification & locations

Identify it by its rounded mudstone body and contrasting cream vein network; acid may fizz on exposed calcite veins. Common sources include Utah and Madagascar, as well as marine sedimentary basins worldwide.