sedimentary

Septarian Nodule

Septarian Concretion (Complex of Calcite, Aragonite, and Limestone/Mudstone)

Hardness: 3.5-4 on Mohs scale; Color: Gray (bentonite mudstone), yellow (calcite), brown (aragonite); Luster: Vitreous to dull; Crystal structure: Trigonal (calcite/aragonite); Cleavage: Perfect rhombohedral in calcite centers.

Hardness: 3
Luster: Vitreous to dull

Identified March 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Cretaceous Period (approx. 50-70 million years ago). They began as sedimentary mud balls that dried and cracked; mineral-rich water then deposited calcite and aragonite into the cracks (septaria).

Uses & applications

Primarily used as decorative display pieces, lapidary material for bookends or spheres, and unique jewelry items. It is highly valued by mineral collectors for its 'dragon skin' appearance.

Geological facts

Also known as 'Dragon Stones.' They are unique because they are a combination of a concretion (the outer shell) and a geode (the hollow or crystal-filled center). Significant deposits are found in Utah and Madagascar.

Field identification & locations

Identify by the distinctive 'cracked' pattern (septaria) where yellow calcite fills dark brown veins in a gray matrix. In the field, look for round, heavy nodules in sedimentary clay beds or along receding shorelines.

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