sedimentary

Polished septarian nodule

Septarian concretion, chiefly calcite and aragonite with clay minerals and iron oxides

AI-generated identification · may be incorrect

Hardness is about 3–4 overall (calcite veins ~3; host ~4–5); brown, black, ochre, and gray with a glossy polish. It has irregular polygonal cracks commonly filled by pale calcite or aragonite; specific gravity is roughly 2.3–2.7.

Identified September 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when shrinkage cracks develop in carbonate-rich mud concretions, later filled by mineral-rich groundwater. Many commercial specimens formed during the Cretaceous, approximately 50–70 million years ago, especially in marine sedimentary settings.

Uses & applications

Used as a lapidary material for polished stones, carvings, cabochons, and decorative objects; generally more collectible than industrially valuable.

Geological facts

The contrasting “dragon egg” pattern results from crack-filling minerals and iron staining. The pictured piece appears tumbled or polished, which obscures the original outer nodule texture.

Field identification & locations

Identify it by its rounded nodule shape, brown mudstone matrix, dark shrinkage cracks, and cream-to-yellow calcite veins; common sources include Madagascar, Utah, and Morocco.