sedimentary
Chert (Septarian Concretion/Nodule)
Microcrystalline silica (SiO2) with carbonate mineral veining
Hardness: 6.5-7 (Mohs scale); Color: Dark gray to brownish-black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.
- Hardness
- 6
- Color
- Dark gray to brownish-black
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Dark gray to brownish-black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.
Formation & geological history
Formed through the precipitation of silica from seawater or subsurface fluids within sedimentary layers. These nodules often develop shrinkage cracks that later fill with minerals like calcite or siderite to create the 'septarian' web pattern.
Uses & applications
Common varieties are used in landscaping, road construction, or as decorative river rock. Higher-quality septarian nodules are cut and polished for jewelry, cabochons, and mineral collecting.
Geological facts
Chert was one of the most important minerals to prehistoric humans because its conchoidal fracture allows it to be shaped into sharp tools like spearheads and scrapers. Septarian concretions are sometimes called 'Dragon Stones' due to their scaly appearance.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel nail) and its pattern of intersecting veins. It is commonly found in riverbeds, glacial tills, and outcrops of limestone or shale.
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