sedimentary
Chert / Flint
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, gray, or black; Luster: Dull to waxy; Structure: Microcrystalline with no visible grains; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Brown, gray, or black
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, gray, or black; Luster: Dull to waxy; Structure: Microcrystalline with no visible grains; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or by the chemical precipitation of silica in sedimentary environments. Common in marine limestone formations.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Today used in construction aggregate, road surfacing, and occasionally as a gemstone (jasper).
Geological facts
Because of its conchoidal fracture, it produces extremely sharp edges, making it the primary material for prehistoric arrowheads and spear points. It is also famous for producing sparks when struck against steel.
Field identification & locations
Identify by its extreme hardness (will scratch glass) and its curved, glass-like fracture surfaces. Often found as nodules within limestone or as pebbles in riverbeds.
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