sedimentary
Chert Concretion
Microcrystalline Quartz (SiO2) in Carbonate Matrix
Hardness: 7 (Mohs), Color: Cream, tan, or grey, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Fracture: Conchoidal
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Physical properties
Hardness: 7 (Mohs), Color: Cream, tan, or grey, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Fracture: Conchoidal
Formation & geological history
Formed by the chemical precipitation of silica within porous sedimentary rocks, typically limestone or chalk, often during early diagenesis as mineral-rich fluids fill voids or replace organic matter.
Uses & applications
Used historically for stone tools due to sharp fracture edges; currently used for landscaping, aggregate, and as a common find for mineral collectors.
Geological facts
Concretions often take on unusual, organic shapes that mimic fossils or body parts, leading to the nickname 'pseudofossils' or 'mimetoliths'.
Field identification & locations
Identified by its extreme hardness (scratches glass) compared to the softer surrounding matrix. Common in riverbeds and limestone outcrops.
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