sedimentary
Chert (often called Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs); Color: Tan, brown, gray, or white; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs)
- Color
- Tan, brown, gray, or white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs); Color: Tan, brown, gray, or white; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from fluids in limestone or chalk beds. Often dated back to the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for stone tools and weapons (arrowheads); currently used in road construction, as a source for glass silica, and as a sparking mechanism in fire-starting tools.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it produces extremely sharp edges used by prehistoric humans for hunting. It is often found as nodules within limestone formations.
Field identification & locations
Identify by its 'waxy' texture and sharp, curved edges where it has broken. It cannot be scratched by a steel knife. Common in limestone outcrops, river beds, and gravel pits.
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