sedimentary
Chert (Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of siliceous remains of marine organisms (diatoms, radiolarians) or by chemical precipitation from silica-rich fluids within limestone or chalk beds.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) and to start fires. Modern uses include road surfacing, jewelry (when colorful/polished), and decorative landscape gravel.
Geological facts
Because it breaks with a conchoidal fracture like glass, it creates edges that are sharp on a molecular level. It has been used by humans for over 2 million years and was the primary material for the Stone Age.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture surfaces. Often found in riverbeds or weathered out of limestone cliffs.
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