sedimentary
Chert (often called Flint or Jasper if reddish)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, tan, and gray with iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Brown, tan, and gray with iron staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, tan, and gray with iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools (arrowheads) due to its sharp edges. Currently used in road construction, as decorative gravel, and occasionally for lapidary work.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture allows it to be shaped into extremely sharp cutting tools. It can also produce sparks when struck against steel.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breakage patterns. Common in gravel pits, riverbeds, and limestone outcrops.
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