sedimentary
Chert (with Calcite Veining)
Silicon Dioxide (SiO2) with Calcium Carbonate (CaCO3) veins
Hardness: 7 (Mohs scale), Color: Dark grey to black with reddish jasperoid patches and white veins, Luster: Waxy to dull, Crystal structure: Microcrystalline/Cryptocrystalline, Fracture: Conchoidal
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark grey to black with reddish jasperoid patches and white veins, Luster: Waxy to dull, Crystal structure: Microcrystalline/Cryptocrystalline, Fracture: Conchoidal
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolaria) on the ocean floor or through chemical precipitation in groundwater. The white veins represent later hydrothermal or tectonic activity where calcite-rich fluids filled fractures.
Uses & applications
Historically used for stone tools (arrowheads). Currently used as construction aggregate, road base, and occasionally for lapidary work in decorative landscaping.
Geological facts
Chert is so hard that it can scratch steel and was one of the primary materials used by prehistoric humans to create sharp cutting edges; the reddish areas are likely due to iron oxide impurities.
Field identification & locations
Identify in the field by its conchoidal fracture (smooth, curved breaks), extreme hardness, and 'clinking' sound when struck. It does not react to acid, though the white calcite veins within it will fizz.
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