sedimentary

Polished Septarian Nodule

Septarian calcite–aragonite concretion, commonly with barite or celestine

AI-generated identification · may be incorrect

Typically tan to brown limestone or calcite matrix cut by dark gray-brown mineral-filled cracks; polished surface has vitreous to waxy luster. Hardness is about 3–4 for calcite/aragonite (up to ~6 for barite), with irregular fracture and no obvious crystal structure externally.

Identified August 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms as a carbonate-rich mud concretion, commonly during the Cretaceous–Tertiary interval, then develops shrinkage cracks that fill with calcite, aragonite, barite, or celestine. Common sources include Madagascar, Utah and Texas, USA, and Morocco.

Uses & applications

Used as ornamental stone, cabochons, paperweights, and geological specimens; attractive pieces are popular in lapidary work and collections.

Geological facts

The dark veins are mineral-filled cracks, producing the characteristic “septarian” pattern; the name derives from Latin septum, meaning partition.

Field identification & locations

Identify it by its rounded nodule form, tan carbonate matrix, and contrasting branching dark seams; vinegar or dilute acid may weakly fizz on exposed calcite. This specimen appears tumbled or polished.