sedimentary
Chert (often called Flint or River Stone)
Chert (Microcrystalline Silicon Dioxide, SiO2)
Hardness: 7 (Mohs scale), Color: tan, brown, and cream, Luster: waxy to dull, Crystal structure: Cryptocrystalline (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, brown, and cream, Luster: waxy to dull, Crystal structure: Cryptocrystalline (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical replacement of limestone by silica-rich fluids. This specimen appears water-worn, likely from a stream or river bed.
Uses & applications
Commonly used as gravel, road fill, and in lapidary work. Historically used for stone tools (arrowheads) due to its sharp conchoidal fracture.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is one of the most common rocks found globally and has been used by humans for hundreds of thousands of years for tool-making.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its waxy texture, and its smooth, curved (conchoidal) breaks. Often found in riverbeds or as nodules within limestone outcrops.
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