sedimentary
Chert (Nodular)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5–7 (Mohs scale), Color: tan, grey, brown with occasional dark inclusions, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
- Hardness
- 6
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Physical properties
Hardness: 6.5–7 (Mohs scale), Color: tan, grey, brown with occasional dark inclusions, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from groundwater filling pores in limestone or chalk, or from the accumulation of silica-rich microfossils. Often found in Phanerozoic Era sedimentary layers.
Uses & applications
Historically used for stone tools (arrowheads/knives) due to sharp edges; currently used for road aggregate, gravel, and occasionally as a source of high-purity silica for manufacturing.
Geological facts
Chert is so hard it can easily scratch glass. It was one of the most vital materials for early humans because its conchoidal fracture allows it to be shaped into razor-sharp edges.
Field identification & locations
Identify by its waxy texture and 'shell-like' curved fractures (conchoidal). It is common in limestone outcrops and river beds. Collectors look for pieces with unique banding or fossils trapped inside.
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