sedimentary

Polished septarian nodule

Septarian concretion, chiefly calcite and aragonite in a clay-rich carbonate matrix

AI-generated identification · may be incorrect

Typically brown to gray matrix with dark angular cracks lined by cream calcite; polished surface has a waxy to vitreous luster. Calcite is Mohs 3, aragonite 3.5–4; the specimen is dense, opaque, and heterogeneous rather than a single crystal.

Identified September 14, 2026More sedimentary →

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Physical properties

Formation & geological history

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

Uses & applications

Used as polished display stone, cabochons, carvings, bookends, and decorative slabs; not generally used structurally because of its variable composition and fracture-filled texture.

Geological facts

The contrasting veins record fluid movement through ancient sediment after the original concretion formed. This example appears tumbled or polished and may include a pale calcite vein.

Field identification & locations

Identify by its polygonal dark matrix and contrasting cream mineral-filled cracks; calcite veins fizz with dilute hydrochloric acid. Common sources include Madagascar, Utah, Morocco, and England.