sedimentary

Polished septarian nodule

Calcite–aragonite–barite septarian concretion, commonly in a claystone or limestone matrix

AI-generated identification · may be incorrect

Typically gray-brown with dark polygonal cracks filled by yellow, white, or brown calcite/barite; polished surfaces are glossy. Matrix hardness is about 3–4 Mohs, while calcite veins are Mohs 3; no consistent crystal structure overall, with granular or fibrous vein crystals.

Identified August 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when a carbonate-rich concretion dries and contracts in marine sediment, producing internal cracks later filled by minerals from groundwater. Many commercial examples formed during the Cretaceous–Tertiary period, roughly 50–100 million years ago.

Uses & applications

Used as a lapidary cabochon, ornamental stone, paperweight, and collector specimen; generally unsuitable for major construction use.

Geological facts

The characteristic “dragon-skin” or polygonal pattern is a network of mineral-filled shrinkage cracks, not biological skin or fossil texture.

Field identification & locations

Identify it by the rounded nodule shape, dark polygonal crack network, and contrasting calcite/barite veins; common sources include Madagascar, Utah, Texas, and Morocco. Avoid harsh acids and prolonged soaking, which can etch or damage calcite.