sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan, brown, and grey with iron staining, Luster: waxy to dull, Crystal structure: cryptocrystalline (microscopic quartz), Cleavage: none (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, brown, and grey with iron staining, Luster: waxy to dull, Crystal structure: cryptocrystalline (microscopic quartz), 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 from groundwater within limestone or chalk beds. Most nodules range from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools/arrowheads due to its sharp conchoidal edges; currently used for landscaping, road aggregate, and as a collector's specimen.
Geological facts
Chert is so hard that it can strike sparks against steel, which led to its historical use as 'flint' for fire-starting and flintlock firearms. Some chert nodules contain excellently preserved micro-fossils.
Field identification & locations
Identify by its 'waxy' texture, extreme hardness (it will scratch glass), and rounded, irregular nodular shape. Common in areas with limestone bedrock or river gravels downstream from such deposits.
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