sedimentary
Chert (Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Grayish-brown; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grayish-brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Grayish-brown; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed as nodules or beds in limestone or chalk through a chemical process where silica replaces carbonate materials, often during diagenesis in marine environments.
Uses & applications
Historically used for stone tools and starting fires; currently used as construction aggregate, road surfacing, and occasionally for lapidary work.
Geological facts
Chert's conchoidal fracture produces extremely sharp edges, which made it the primary material for arrowheads and spear points for thousands of years throughout human history.
Field identification & locations
Identify by its extreme hardness (will scratch glass), waxy feel, and characteristic curved (conchoidal) breaks. Extremely common in stream beds and areas with limestone outcrops.
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