sedimentary
Chert (Nodule)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale; Color: Dark brown, black, or grey (often due to organic impurities); Luster: Waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 7 on the Mohs scale
- Color
- Dark brown, black, or grey (often due to organic impurities)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on the Mohs scale; Color: Dark brown, black, or grey (often due to organic impurities); Luster: Waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the precipitation of silica-rich fluids within sedimentary host rocks like limestone or chalk. Many specimens date from the Paleozoic or Mesozoic eras, originating from the accumulation of siliceous skeletal remains (diatoms/radiolarians) on the seafloor.
Uses & applications
Historically used for stone tools (arrowheads/knives) due to sharp edges; currently used as road aggregate, in glass manufacturing, and as ornamental stones for lapidary work.
Geological facts
Because of its extreme hardness and conchoidal fracture, chert was one of the most important minerals for early humans to create fire-starting tools and weaponry. It is a very durable rock that often survives long after its limestone host rock has eroded away.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass easily. It is often found as rounded nodules in stream beds or protruding from limestone cliffs. Common worldwide, notoriously in the Midwestern USA and Southern England.
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