sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, tan, gray; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Brown, tan, gray
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, tan, gray; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine or groundwater environments. These nodules often displace limestone or chalk over millions of years.
Uses & applications
Historically used for stone tools (flintknapping); currently used in road construction, as an abrasive, and small polished pieces are used in decorative gravel or folk art.
Geological facts
Chert is so hard it can strike sparks against steel. It was the primary material for prehistoric arrowheads and spear tips because of its predictable, razor-sharp fracturing pattern.
Field identification & locations
Identify in the field by its extreme hardness (scratches glass) and smooth, shell-like (conchoidal) fracture surfaces when broken. Common in limestone outcrops and riverbeds. Collectors look for unique shapes or 'geode' interiors.
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