sedimentary

Polished septarian nodule

Septarian concretion, commonly calcite/aragonite with brown iron-oxide or siderite veins in a clay-rich matrix

AI-generated identification · may be incorrect

Typically dark gray to brown, rounded, and polished, with contrasting tan/yellow fracture-filling veins; hardness about 3–4 (matrix may be softer), dull to waxy luster, and no consistent cleavage in the whole nodule.

Identified August 16, 2026More sedimentary →

Explore Polished septarian nodule in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Forms when a mudstone or shale concretion shrinks and cracks during burial, followed by mineral-rich groundwater depositing calcite, aragonite, and iron minerals in the fractures. Most examples formed during the Cretaceous to Eocene, roughly 145–34 million years ago.

Uses & applications

Used as polished display stones, cabochons, spheres, and lapidary material; it has modest decorative and collecting value rather than major industrial use.

Geological facts

The name derives from Latin septum, meaning “partition,” referring to its crack network. Vein colors can indicate changing groundwater chemistry during burial.

Field identification & locations

Identify it by a rounded dark host crossed by angular tan or golden veins, often with a polished surface; common sources include Utah and Madagascar. The image appears consistent with a polished septarian nodule, though exact mineralogy needs testing.