sedimentary
Chert (varieties include Flint and Jasper)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, beige, grey, or white; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, beige, grey, or white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, beige, grey, or white; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the replacement of calcium carbonate in limestone. It occurs in nodules or bedded deposits within sedimentary rock layers.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its conchoidal fracture. Today used as road surfacing, in construction, and as a raw material for silicon production.
Geological facts
Because it breaks with very sharp edges, chert was one of the most important minerals for early human survival, used for both weapons and starting fires by striking against steel or pyrite.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by steel) and curved, glass-like fracture surfaces. Often found as rounded nodules protruding from limestone or as sharp, weathered fragments in creek beds.
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