sedimentary

Chert (Flint)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness of 7 on the Mohs scale. Typically grey, black, or tan with a waxy to dull luster. Exhibits a characteristic conchoidal fracture. Microscopic crystal structure with no visible cleavage and a specific gravity of 2.65.

Identified March 17, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation of silica-rich organic remains such as radiolarians or diatoms on the seafloor, or through chemical precipitation in groundwater. Occurs throughout geological history from the Precambrian to present, often as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons (arrowheads, axes) due to its sharp edges. Modernly used in construction aggregate, road surfacing, and as a component in certain ceramics. Occasionally used for jewelry in its colorful forms like Jasper.

Geological facts

Flint was a vital survival resource for prehistoric humans; it was used not only for tools but also to create sparks for starting fires when struck against steel or pyrite. Large deposits are found in the White Cliffs of Dover.

Field identification & locations

Identify in the field by its hardness (will scratch glass), waxy feel, and smooth, curved shell-like fractures. It often has a white weathered outer rind (cortex). Common in sedimentary basins and riverbeds worldwide.

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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