sedimentary
Chert (Flint Nodules)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brown, tan, and dark grey mottling, 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: Brown, tan, and dark grey mottling, Luster: Dull to waxy, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed primarily in marine environments through the accumulation of silica-rich organic remains or chemical precipitation within limestone/chalk beds. These nodules often date back to various geologic periods including the Cretaceous and Carboniferous.
Uses & applications
Historically used for stone tools/weapons due to sharp edges; currently used for lapidary work, landscaping, and primitive fire-starting (flint and steel).
Geological facts
Chert has been used by humans for over 2 million years. The dark grey areas within the tan cortex are often indicative of high organic content or different mineralization phases.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel nail), smooth waxy feel on broken surfaces, and distinctive curved 'conchoidal' fracture patterns. Common in riverbeds and limestone outcrops.
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