sedimentary
Septarian Nodule
Siderite or Calcite Concretion
Hardness: 3.5-4 (Mohs), Color: brown, yellow, and grey, Luster: dull to greasy, Structure: microcrystalline with crystalline veins, Cleavage: rhombohedral in centers.
- Hardness
- 3
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Physical properties
Hardness: 3.5-4 (Mohs), Color: brown, yellow, and grey, Luster: dull to greasy, Structure: microcrystalline with crystalline veins, Cleavage: rhombohedral in centers.
Formation & geological history
Formed about 50-70 million years ago during the Cretaceous period from mud balls of decomposing marine life and clay that dehydrated and cracked, later filling with mineral deposits like calcite.
Uses & applications
Primarily used for ornamental purposes, lapidary work, jewelry (cabochons), and as display specimens for collectors.
Geological facts
The name comes from the Latin word 'septum' meaning partition, referring to the cracks that divide the nodule. They are often called 'Dragon Stones' in folklore.
Field identification & locations
Identified by their unique 'turtle back' patterns or cracked internal cavities filled with yellow calcite. Commonly found in Utah, USA and Madagascar. Look for heavy, rounded nodules in shale beds.
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