sedimentary
Chert (Flinty variety)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Grey to dark grey with waxy luster; Crystal structure: Trigonal (microscopic); Fracture: Conchoidal; Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Grey to dark grey with waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Grey to dark grey with waxy luster; Crystal structure: Trigonal (microscopic); Fracture: Conchoidal; Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or chemical precipitation from groundwater. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and starting fires; currently used in decorative landscaping, as an abrasive, and occasionally in jewelry as cabochons.
Geological facts
Chert is so hard it can scratch steel. When broken, it creates extremely sharp edges, which is why it was the primary material for prehistoric arrowheads and knives.
Field identification & locations
Identify by its waxy appearance, extreme hardness, and smooth, curved (conchoidal) fracture surfaces. Commonly found in riverbeds or eroded from limestone outcroppings.
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