sedimentary
Chert (often called Flint when dark)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: light gray to dark gray; Luster: waxy to dull; Crystal Structure: Microcrystalline; Cleavage: none (shows conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- light gray to dark gray
- Luster
- waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: light gray to dark gray; Luster: waxy to dull; Crystal Structure: Microcrystalline; Cleavage: none (shows conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like diatoms or radiolarians) in seafloor sediments. Most date from the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) and fire-starting. Modern uses include road surfacing, abrasives, and decorative landscaping.
Geological facts
Chert breaks with a conchoidal fracture, creating sharp edges like glass; because of this, it was the primary material for the Stone Age technology. It is often found as nodules inside limestone or chalk formations.
Field identification & locations
Identify by its 'shell-like' curved fracture surfaces and inability to be scratched by a steel knife. Common worldwide in limestone beds. Collectors look for unique colors or evidence of 'knapping' (shaping by humans).
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