sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Varying shades of gray, brown, and tan with a yellowish weathered cortex; Luster: Waxy to dull; Structure: Microcrystalline or cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Varying shades of gray, brown, and tan with a yellowish weathered cortex; Luster: Waxy to dull; Structure: Microcrystalline or cryptocrystalline; 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 from groundwater within limestone or chalk beds. Often represents Cretaceous or older geological formations.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used for road ballast, construction aggregate, and occasionally in jewelry as tumble-polished stones.
Geological facts
Chert is so hard it can strike sparks against steel, a property that led to its use in flintlock firearms. It is chemically identical to quartz but has a different microscopic crystal structure.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds or weathered out of carbonate rock outcrops.
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