sedimentary
Chert (and/or Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, green, tan, or brown; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, green, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, green, tan, or brown; Luster: Waxy to dull; Structure: Microcrystalline; 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 through the chemical precipitation of silica in marine environments. It is often found as nodules or layers within limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrows, scrapers) due to its sharp edges. Modern uses include road surfacing, as an ingredient in ceramics, and occasionally as a low-cost gemstone when colorful.
Geological facts
Chert has been used by humans for over a million years. When struck against steel, it can produce sparks, which made it a primary material for 'flintlock' firearms.
Field identification & locations
Identify in the field by its waxy texture and its characteristic curved, shell-like (conchoidal) fracture pattern when broken. It cannot be scratched by a steel knife. Common in limestone outcrops worldwide.
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