sedimentary

Chert (Iron-rich variety)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 6.5–7 on Mohs scale. Color: Brown, reddish-brown, or dark grey. Luster: Dull to waxy. Crystal Structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific gravity: 2.6–2.7.

Hardness: 6
Color: Brown, reddish-brown, or dark grey
Luster: Dull to waxy

Identified March 17, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in sedimentary basins. These rocks can range from Precambrian to Cenozoic in age.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modern uses include road surfacing, decorative gravel, and occasionally as semi-precious gemstones (Jasper).

Geological facts

Chert is so hard it can strike sparks against steel, a property used by ancient people to start fires. Variations in color are usually caused by trace elements; brown/red hues often indicate iron oxide content.

Field identification & locations

Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found in limestone or chalk beds as nodules or layers. Common globally, especially in areas with ancient seafloors.

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Sandstone or Siltstone

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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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