sedimentary
Septarian Nodule
Septarian Concretion (CaCO3 with Siderite/Calcite/Aragonite)
Hardness: 3.5-4.0 (Mohs scale); Color: Brown, yellow, grey, white; Luster: Vitreous to dull; Crystal structure: Trigonal (Calcite) or Orthorhombic (Aragonite); Specific gravity: 2.7-2.9.
- Hardness
- 3
- Color
- Brown, yellow, grey, white
- Luster
- Vitreous to dull
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Physical properties
Hardness: 3.5-4.0 (Mohs scale); Color: Brown, yellow, grey, white; Luster: Vitreous to dull; Crystal structure: Trigonal (Calcite) or Orthorhombic (Aragonite); Specific gravity: 2.7-2.9.
Formation & geological history
Formed during the Cretaceous period (50-70 million years ago) when volcanic eruptions killed marine life that sank to the ocean floor. Cracks formed as the organic-rich mudballs dried and shrank, which were later filled with mineral deposits like calcite.
Uses & applications
Used primarily for ornamental purposes, jewelry (cabochons), interior design (bookends, spheres), and valued as a unique specimen by geological collectors.
Geological facts
Septarian nodules are also known as Dragon Stones due to their cracked, scaly appearance. The name comes from the Latin 'septum' meaning partition, referring to the cracks that divide the concretion.
Field identification & locations
Identify by its distinctive pattern of radiating cracks (septaria) filled with yellow calcite and dark siderite/clay. Commonly found in Utah, USA, and Madagascar. Look for round mud-like nodules in shale beds.
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