Rock Identifier
Septarian Nodule (Septarian Concretion (Calcium Carbonate/Aragonite/Calcite/Siderite)) — sedimentary
sedimentary

Septarian Nodule

Septarian Concretion (Calcium Carbonate/Aragonite/Calcite/Siderite)

Hardness (Mohs scale): 3.5 to 4; Color: Brown, tan, and gray with yellow/white veins; Luster: Waxy to dull matrix with vitreous crystalline veins; Crystal structure: Trigonal/Orthorhombic veins within an amorphous sedimentary matrix; Cleavage: Perfect rhombohedral in calcite veins; Specific gravity: 2.6 - 2.8

Color
Brown, tan, and gray with yellow/white veins
Luster
Waxy to dull matrix with vitreous crystalline veins
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Physical properties

Hardness (Mohs scale): 3.5 to 4; Color: Brown, tan, and gray with yellow/white veins; Luster: Waxy to dull matrix with vitreous crystalline veins; Crystal structure: Trigonal/Orthorhombic veins within an amorphous sedimentary matrix; Cleavage: Perfect rhombohedral in calcite veins; Specific gravity: 2.6 - 2.8

Formation & geological history

Formed during the Cretaceous period (50 to 70 million years ago) in seafloor sediments. As the clay balls dried and shrank, they cracked, and mineral-rich waters (bearing calcite or aragonite) filled the voids, crystallizing over time.

Uses & applications

Primarily used as a lapidary material for decorative carvings, bookends, jewelry, and as highly prized collector display specimens.

Geological facts

The name comes from the Latin word 'septum', meaning partition, referring to the cracks that divide the concretion into segments. They are often called 'Dragon Stones' due to their scaly appearance when cut and polished.

Field identification & locations

Identify by looking for a characteristic spiderweb or 'tortoiseshell' pattern of light-colored veins cutting through a darker, mudstone or ironstone matrix. Often found in dry lake beds or eroded sedimentary hillsides with clay-rich soil.

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