sedimentary
Chert (with Iron Staining)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Tan to grey with reddish-brown iron patina, Luster: Dull to waxy, Crystal structure: Microcrystalline (Cryptocrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Tan to grey with reddish-brown iron patina, Luster: Dull to waxy, Crystal structure: Microcrystalline (Cryptocrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the ocean floor or through chemical precipitation in groundwater. This specimen shows oxidation (rusting) from exposure to water and air.
Uses & applications
Historically used for making sharp tools like arrowheads and scrapers. Currently used as road aggregate, in landscaping, and as a source of silica in glass manufacturing.
Geological facts
Chert is so hard it can strike sparks against steel. It is chemically identical to quartz but has a much finer, denser internal structure. This specific piece is a river stone that has undergone significant chemical weathering.
Field identification & locations
Identify in the field by its very high hardness (it will scratch glass and steel) and its conchoidal (shell-like) fracture edges. Commonly found in river beds and sedimentary layers worldwide.
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