sedimentary
Chert Nodule (Commonly known as Flint or River Rock)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, tan, and white; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Brown, tan, and white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, tan, and white; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through chemical precipitation from silica-rich fluids within sedimentary host rocks (like limestone or chalk). This specimen appears to be a stream-rolled nodule, likely from the Cretaceous or Paleozoic eras depending on local geology.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used in road construction, landscaping, and as a durable abrasive in rock tumbling.
Geological facts
Chert is so durable that it often survives much longer than the host rock it formed in, leading to rounded 'river rocks' like this one found far from their original source. It was the primary 'high-tech' material for prehistoric humans for crafting arrowheads and knives.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel knife) and waxy, smooth texture. Look for conchoidal (shell-like) fractures on broken edges. Common in riverbeds and limestone outcrops.
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