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
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Physical properties
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.
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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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone
Arenite (primarily SiO2)
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