sedimentary
Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brown, tan, or grey, Luster: Waxy to dull, Crystal structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, tan, or grey, Luster: Waxy to dull, Crystal structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed primarily by the accumulation of siliceous remains of organisms like diatoms or radiolarians, or through chemical precipitation in shallow marine environments. Most specimens range from Precambrian to Cenozoic in age.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) and as fire-starters. Today it is used in construction aggregate and as a semi-precious stone for lapidary work.
Geological facts
Chert has a conchoidal fracture which allows it to be shaped into razor-sharp edges. It is often referred to as flint when found in chalk or marly limestone.
Field identification & locations
Identify by its extreme hardness (scratches glass) and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone beds or as loose nodules in riverbeds.
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Sandstone or Siltstone
Arenite (if sandstone)
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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