sedimentary

Chert (containing Flint nodules)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, brown, black, and tan (waxy patina when wet); Luster: Greasy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, brown, black, and tan (waxy patina when wet)
Luster: Greasy to vitreous

Identified March 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica in marine environments, often replacing carbonate material (limestone or chalk) through diagenesis. Can range from Precambrian to recent geological ages.

Uses & applications

Historically used for stone tools (arrowheads). Modern uses include road surfacing, railroad ballast, and gemstone tumbling for jewelry.

Geological facts

Chert and flint were arguably the first 'industrial' materials used by early humans due to their ability to produce razor-sharp edges when fractured. Flint is specifically the dark, nodular variety found in chalk beds.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture patterns, extreme hardness (scratches glass), and waxy appearance. Commonly found in limestone outcrops or as river gravel.

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

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Sandstone (with man-made markings)

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