sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Dull to waxy; Structure: Cryptocrystalline/Amorphous appearance; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Dull to waxy; Structure: Cryptocrystalline/Amorphous appearance; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation within limestone or chalk beds. Often found in Phanerozoic era strata.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracturing. Currently used as road aggregate, in glass manufacturing, and as a decorative landscaping stone.
Geological facts
Chert nodules are frequently mistaken for dinosaur eggs or fossils due to their rounded, organic shapes. They are extremely resistant to weathering and often remain long after the surrounding limestone has eroded.
Field identification & locations
Identify in the field by its hardness (will scratch glass) and its smooth, shell-like (conchoidal) fracture when broken. Found globally in sedimentary basins, particularly in the Midwestern USA and Southern UK.
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