sedimentary
Chert (Fossiliferous)
Silicon dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan/cream/brown, Luster: waxy to dull, Crystal structure: microcrystalline (cryptocrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan/cream/brown, Luster: waxy to dull, Crystal structure: microcrystalline (cryptocrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed by the precipitation of silica in sedimentary environments, often replacing organic material or filling voids in limestone. This specimen appears to contain fossilized imprints or casts, likely from the Paleozoic era.
Uses & applications
Abrasives, construction aggregate, historically used for stone tools (arrowheads), and popular for hobbyist collecting due to fossil content.
Geological facts
Chert is so hard it can scratch steel and was once used extensively by prehistoric humans to start fires and make sharp tools. Many Paleozoic cherts contain preserved remains of sea sponges or bryozoans.
Field identification & locations
Identify in the field by its hardness (will scratch glass), conchoidal (shell-like) fracture patterns, and waxy texture when broken. Often found in riverbeds or limestone outcrops.
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