sedimentary
Chert / Flint Nodule
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within sedimentary rock layers, often limestone or chalk. Many originate from the accumulation of microscopic skeletal remains of silica-secreting organisms such as diatoms or radiolarians. Age ranges from Precambrian to Cenozoic.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when broken. Also used in high-end road construction, decorative gravel, and historically as a strike-stone for fire-starting.
Geological facts
Chert and flint are essentially the same material; the term 'flint' is usually reserved for dark nodules found in chalk. It is so tough and chemically stable that it often remains on the surface long after the surrounding limestone has eroded away.
Field identification & locations
Identify in the field by its smooth, glass-like fracture (conchoidal) and its ability to scratch glass or steel. Often found as rounded or irregular nodules in limestone outcroppings or creek beds. Look for a waxy texture compared to surrounding rocks.
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