sedimentary
Chert (River Pebble)
Chert (Microcrystalline Silicon Dioxide, SiO2)
Hardness: 7 (Mohs), Color: Brownish-tan with white weathered patches, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal to irregular, Specific Gravity: 2.5-2.8
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Physical properties
Hardness: 7 (Mohs), Color: Brownish-tan with white weathered patches, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal to irregular, Specific Gravity: 2.5-2.8
Formation & geological history
Formed via the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation from silica-rich fluids in sedimentary basins. The rounded shape indicates transport in a high-energy water environment, such as a riverbed.
Uses & applications
Historically used for stone tools (flint knapping) due to its sharp fracture edges. Modernly used as construction aggregate, road base, and occasionally as decorative river rock in landscaping.
Geological facts
Chert is so hard it can scratch steel. It is one of the most common materials used by prehistoric humans for creating arrowheads and scrapers. Its brown color is often due to iron oxide impurities.
Field identification & locations
Identify by its hardness (will scratch glass) and smooth, water-worn texture. Commonly found in riverbeds, gravel pits, and coastal areas. Collectors look for pieces with unique banding or fossils.
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