sedimentary
Chert (Iron-stained)
Chert / Microcrystalline Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-red to tan due to iron oxidation, Luster: Dull to waxy, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-red to tan due to iron oxidation, Luster: Dull to waxy, Crystal structure: Cryptocrystalline, 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. Often represents ancient seafloor environments.
Uses & applications
Historically used for stone tools/arrowheads. Currently used in road construction aggregate, as a component in ceramics, and as landscaping stone.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is virtually identical to flint, though 'flint' is usually reserved for dark grey or black specimens found in chalk.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture patterns and the fact that it cannot be scratched by a steel knife. Common in riverbeds and limestone outcrops worldwide.
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