sedimentary

Polished septarian nodule fragment

Septarian calcite–aragonite concretion, commonly with iron-rich minerals and possibly barite

AI-generated identification · may be incorrect

Brown to gray, polished surface with a network of dark cracks filled by pale cream minerals; dull to waxy luster. Calcite hardness 3, aragonite 3.5–4, with irregular fracture and variable density.

Identified August 19, 2026More sedimentary →

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Physical properties

Brown to gray, polished surface with a network of dark cracks filled by pale cream minerals; dull to waxy luster. Calcite hardness 3, aragonite 3.5–4, with irregular fracture and variable density.

Formation & geological history

Forms as a sedimentary concretion when shrinkage cracks develop in a carbonate-rich mud nodule; later mineral-rich fluids fill the cracks. Most commercial septarian material formed during the Cretaceous–Tertiary interval, roughly 65–100 million years ago.

Uses & applications

Used as a decorative stone, cabochon, polished pebble, and collector specimen; too soft and irregular for most durable construction applications.

Geological facts

The distinctive crack pattern is called septaria. Yellow or cream veins are usually calcite or aragonite, while brown coloration commonly comes from iron oxides.

Field identification & locations

Identify it by its rounded nodule texture, brown matrix, and contrasting polygonal vein network; common sources include Utah and Madagascar. Acid may weakly fizz on exposed calcite, but testing can damage the polish.