sedimentary
Septarian Nodule
Septarian Nodule (Calcite-filled Concretion)
Hardness: 3.5 to 4 (calcite portions), Color: Dark grey/brown matrix with yellow/tan veins, Luster: Dull to waxy, Crystal Structure: Trigonal (calcite centers), Cleavage: Perfect rhombohedral in centers, Specific Gravity: 2.7-2.9
- Hardness
- 3
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Physical properties
Hardness: 3.5 to 4 (calcite portions), Color: Dark grey/brown matrix with yellow/tan veins, Luster: Dull to waxy, Crystal Structure: Trigonal (calcite centers), Cleavage: Perfect rhombohedral in centers, Specific Gravity: 2.7-2.9
Formation & geological history
Formed during the Cretaceous period (50-70 million years ago) in marine environments. Clay masses became hardened into concretions; as they dried, they shrunk and cracked, with calcite later seeping into and filling the cracks.
Uses & applications
Primarily used for lapidary work, decorative display, ornamental carvings, and as a popular collector's item due to its unique 'dragon skin' patterns.
Geological facts
Also known as 'Dragon Stones' due to their scaly appearance. The name comes from the Latin word 'septum', meaning a partition or wall, referring to the cracks that divide the nodule.
Field identification & locations
Identify by the distinctive angular 'turtle-shell' vein patterns on the exterior or interior. Often found in dry lake beds or along eroding cliffs in places like Utah or Madagascar.
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