sedimentary
Chert (Nodule)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or light grey; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or light grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or light grey; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica from groundwater within limestone or chalk beds. This specific specimen is a water-worn nodule, likely formed during the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for making stone tools (arrowheads). Modern uses include road surfacing, decorative garden stones, and as an industrial abrasive.
Geological facts
Chert is so hard it can create sparks when struck against steel. Nodules often take on bulbous, 'bone-like' or 'skull-like' shapes due to the way silica replaced organic or carbonate material.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, waxy feel. Found worldwide in sedimentary basins, riverbeds, and coastal areas.
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