sedimentary
Chert (v. Flint)
Sedimentary Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Cream, tan, grey, or brown; Luster: Dull to waxy; Structure: Cryptocrystalline/Massive; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Cream, tan, grey, or brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Cream, tan, grey, or brown; Luster: Dull to waxy; Structure: Cryptocrystalline/Massive; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like diatoms or sponges) on the seafloor. This specimen appears to be a weathered river pebble or nodule, likely millions of years old.
Uses & applications
Historically used for making stone tools (arrowheads). Modern uses include road aggregate, decorative landscaping, and occasionally in the ceramic and glass industries.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it produces very sharp edges. It was the primary material for survival tools in the Stone Age. It produces sparks when struck against steel.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel nail) and smooth, curved fracture surfaces. Found globally in riverbeds, limestone outcrops, and coastal areas. Collectors look for unique colors or fossil inclusions.
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