sedimentary
Chert (often called Flint when dark)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically grey, brown, or tan; Luster: waxy to dull; Structure: microcrystalline (no visible grains); Cleavage: none, breaks with conchoidal (shell-like) fracture; Specific Gravity: 2.6
- Hardness
- 7 on Mohs scale
- Color
- typically grey, brown, or tan
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically grey, brown, or tan; Luster: waxy to dull; Structure: microcrystalline (no visible grains); Cleavage: none, breaks with conchoidal (shell-like) fracture; Specific Gravity: 2.6
Formation & geological history
Formed by the precipitation of silica-rich fluids in sedimentary environments. Often creates nodules within limestone or chalk beds. Many specimens originate from the accumulation of silica-based plankton (diatoms or radiolarians) on the prehistoric ocean floor.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when broken. Current uses include road surfacing, jewelry (when colorful), and industrial abrasives.
Geological facts
Chert is so hard it can strike sparks from steel, which led to its use in 'flintlock' firearms. It is chemically very stable and can survive millions of years of erosion, often being found in stream beds long after the surrounding limestone has dissolved.
Field identification & locations
Identify by its 'glassy' smooth texture on broken surfaces and its ability to scratch glass easily. Look for it in gravel bars, river beds, or embedded as rounded modules in limestone outcrops. Collectors look for 'Lake Superior Agates' or high-quality Jasper (varieties of chert).
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