sedimentary
Chert Nodule (River Cobble)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, blue-gray, tan with iron staining (rust-brown); Luster: Waxy to dull/earthy; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, blue-gray, tan with iron staining (rust-brown)
- Luster
- Waxy to dull/earthy
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, blue-gray, tan with iron staining (rust-brown); Luster: Waxy to dull/earthy; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within voids or replacing sections of carbonate rocks like limestone. Many date back hundreds of millions of years (Paleozoic or Mesozoic era). The rounded shape indicates extensive water erosion and transport in a riverbed.
Uses & applications
Historically used for stone tools and arrowheads. Currently used as construction aggregate, landscaping, road base, and occasionally as a source for polishing into decorative 'river stones'.
Geological facts
Chert is so hard it can strike sparks from steel, which led to its use in flintlock firearms. Darker varieties are often referred to as flint. This specific specimen shows 'desert varnish' or water-worn rounding, suggesting it has been tumbler-polished by nature over thousands of years.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a knife) and smooth, rounded surface indicating river transport. It often features a white or tan weathered 'rind' (cortex). Found globally in riverbeds and areas with ancient limestone deposits.
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