sedimentary

Polished septarian nodule fragment

Septarian calcite–aragonite concretion in clay-rich limestone/mudstone

AI-generated identification · may be incorrect

Brown, fine-grained matrix cut by cream-white calcite veins; typically dull to subtly waxy, with no obvious crystals. Matrix hardness is about Mohs 3–4, while calcite veins are Mohs 3 and fizz in dilute acid; specific gravity roughly 2.3–2.6.

Identified August 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when a carbonate-rich mud concretion dries and contracts, creating cracks later filled by calcite or aragonite. Septarian nodules commonly formed during the Cretaceous–Paleogene, though age varies by locality.

Uses & applications

Used as polished cabochons, ornamental stones, and collector specimens; not generally used for structural construction.

Geological facts

The characteristic polygonal vein network records shrinkage and later mineral precipitation. The brown color commonly comes from iron-bearing clay or iron oxides.

Field identification & locations

Identify by a rounded brown host rock crossed by contrasting cream calcite veins; acid effervescence and a soft matrix support the identification. Common sources include Utah and Madagascar, among others.