$Polished septarian nodule (Septarian concretion; calcite-filled fractures in a clay-rich carbonate matrix) — sedimentary$

sedimentary

Polished septarian nodule

Septarian concretion; calcite-filled fractures in a clay-rich carbonate matrix

AI-generated identification · may be incorrect

Typically dark gray to black matrix with contrasting cream calcite veins; Mohs hardness about 3 for calcite and 3–5 for the matrix, earthy to waxy luster, irregular granular structure, and no consistent cleavage in the whole specimen.

Identified August 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when carbonate-rich mud concretions crack during burial and later fractures fill with calcite, aragonite, or barite. Many commercial examples formed during the Cretaceous, roughly 50–100 million years ago, especially in marine sediments.

Uses & applications

Used as polished display stones, cabochons, and lapidary objects; rarely used industrially because specimens are small and decorative.

Geological facts

The branching vein pattern is called septaria and reflects shrinkage or pressure-related cracking inside the concretion.

Field identification & locations

Commonly found in Utah and Madagascar, as well as sedimentary basins worldwide. Identify it by the dark rounded nodule, polygonal pale veins, and strong contrast; acid may fizz on exposed calcite.