sedimentary
Chert (often called Flint when dark)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed from the chemical precipitation of silica-rich fluids or the accumulation of silica-based micro-fossils (like radiolarians or diatoms) in marine environments. This specimen likely originates from Paleozoic to Mesozoic sedimentary beds.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its sharp conchoidal edges; currently used for road ballast, friction-based ignition (flint and steel), and as a landscaping stone.
Geological facts
Chert is so hard it can scratch steel. Because it breaks with a razor-sharp edge, it was the primary technology for tool-making for millions of years of human history.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture patterns and inability to be scratched by a steel knife. Commonly found in limestone outcroppings or as river pebbles. Key for field identification is the smooth, waxy feel of the broken surface.
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