sedimentary
Chert (specifically Jasper or Flint variant)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, reddish-brown, or dark grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Brown, reddish-brown, or dark grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, reddish-brown, or dark grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation from silica-rich fluids in sedimentary basins. These rocks can date from the Archean to the Cenozoic.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used in jewelry (as jasper), as a construction aggregate, and for landscaping.
Geological facts
Due to its conchoidal fracture, chert breaks with very sharp edges, making it the primary material for prehistoric arrowheads and spear points. It is virtually insoluble in most chemicals except hydrofluoric acid.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture, its inability to be scratched by a steel knife, and its waxy appearance. Commonly found in limestone or chalk beds as nodules or layers.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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