sedimentary
Chert (Nodular or Fossiliferous)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale); Color: Typically gray, brown, or black; Luster: Dull to waxy; Structure: Microcrystalline with conchoidal (shell-like) fracture; Cleavage: None; Specific Gravity: 2.6 – 2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Typically gray, brown, or black
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Typically gray, brown, or black; Luster: Dull to waxy; Structure: Microcrystalline with conchoidal (shell-like) fracture; Cleavage: None; Specific Gravity: 2.6 – 2.7.
Formation & geological history
Formed through the precipitation of silica-rich fluids in limestone or chalk beds, or by the accumulation of siliceous organic remains (like diatoms or radiolarians). Formation can range from the Precambrian to late Cenozoic era.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges. Modernly used as road aggregate, in concrete, and for laboratory grinding equipment. Polished varieties are used in decorative stones/jewelry.
Geological facts
Chert is extremely durable and chemically stable. Many ancient civilizations used it to spark fires (strike against steel). Some specimens contain perfectly preserved prehistoric fossils that are invisible to the naked eye.
Field identification & locations
Identify in the field by its hardness (it will easily scratch glass) and its smooth, waxy surface on broken edges. It is globally common, often found as nodules or 'lumps' within sedimentary outcrops. Collectors look for unique banding or fossil inclusions.
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