sedimentary
Chert Concretion with Iron Oxide Staining
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Tan, brown, and grey with rusty iron staining; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Tan, brown, and grey with rusty iron staining
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 Mohs scale; Color: Tan, brown, and grey with rusty iron staining; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica within sedimentary rock layers, often replacing organic material or filling voids. The exterior displays oxidation (rusting) from exposure to water and oxygen.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers); currently used for landscaping, road base, and as a collector's specimen.
Geological facts
Chert is so hard it can strike sparks against steel. The nodular shape is typical of chemical precipitation in limestone or shale beds.
Field identification & locations
Identify by its waxy luster on fresh breaks, extreme hardness (cannot be scratched by a steel knife), and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds or eroded limestone outcrops.
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