sedimentary
Chert (variations include Flint or Jasper)
Chert (chemical sedimentary rock), Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, brown, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, brown, or black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, brown, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of siliceous remains (diatoms or radiolarians) or by the precipitation of silica from groundwater within limestone or chalk beds. Often Cretaceous to Tertiary in age but found in layers of all ages.
Uses & applications
Historically used for stone tools and weapons due to sharp conchoidal edges; currently used for road surfacing, gravel, and primitive fire-starting (flint).
Geological facts
Chert is so hard it can scratch glass. It was one of the first materials traded by humans over long distances in the Stone Age because of its predictable fracturing patterns.
Field identification & locations
Identify by its waxy luster, extreme hardness, and 'shell-like' (conchoidal) curved fracture lines when broken. Commonly found in nodules within limestone or as river pebbles.
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