sedimentary
Chert (Black Chert or Flint)
Silicon Dioxide (SiO2)
Hardness: 7 Mohs, Color: Deep grey to black, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 Mohs, Color: Deep grey to black, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in sedimentary environments. Often found as nodules or layers in limestone or chalk beds.
Uses & applications
Historically used for stone tools and starting fires (striking against steel). Modernly used in construction aggregate, road surfacing, and as gemstone material for lapidary work.
Geological facts
Black chert is often referred to as 'Flint' when found in chalk or marl. It fractures into extremely sharp edges, which made it the primary material for prehistoric arrowheads and knives.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture and the ability to scratch glass. It will produce sparks when struck against carbon steel. Common in limestone outcrops worldwide.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone
Arenite (primarily SiO2)
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