sedimentary

Chert (Jasper/Flint variety)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 on the Mohs scale. Color: Tan, brown, or grey with subtle banding. Luster: Dull to waxy. Structure: Trigonal/Microcrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.

Hardness: 6
Color: Tan, brown, or grey with subtle banding
Luster: Dull to waxy

Identified March 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from hydrothermal fluids in sedimentary basins. These stones typically date from the Precambrian to present day.

Uses & applications

Historically used for tools and weapons due to its sharp edges; currently used as construction aggregate, in road surfacing, and as semi-precious gemstones for jewelry (cabs or tumbling).

Geological facts

Chert was one of the first materials used by early humans for tool-making because it breaks in a predictable 'conchoidal' pattern, creating razor-sharp edges. It is often found as nodules in limestone or chalk beds.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture surfaces. It feels much smoother and denser than common sandstone. Common in riverbeds and limestone outcrops.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

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Sandstone (with man-made markings)

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