sedimentary
Chert (variations include Flint/Jasper)
Chert (SiO2)
Hardness: 7 (Mohs scale), Color: light gray to tan (can be multicolored), Luster: waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline quartz, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: light gray to tan (can be multicolored), Luster: waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline quartz, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from groundwater within limestone or chalk beds. Most common in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to conchoidal fracturing. Modernly used in construction aggregate, road base, and as a raw material for ceramics.
Geological facts
Because it breaks with very sharp edges, chert was one of the most vital resources for early humans. It is chemically very stable and resistant to weathering, which is why it often remains as pebbles in gravel pits after other rocks have eroded.
Field identification & locations
Identify in the field by its hardness (it will scratch glass and steel) and its distinct shell-like (conchoidal) curved fracture lines. Commonly found in stream beds, gravel deposits, and as nodules within limestone outcrops.
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