sedimentary
Chert (often called Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish, tan, or grey, Luster: Waxy to dull, Crystal Structure: Hexagonal (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish, tan, or grey, Luster: Waxy to dull, Crystal Structure: Hexagonal (microscopic), 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 radiolarians) or through the chemical replacement of host rocks like limestone. Common in nodules or bedded deposits throughout the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Today used in road construction, as an abrasive, and sometimes in landscaping.
Geological facts
Chert has a distinctive conchoidal (shell-like) fracture that creates sharp edges, which is why it was the primary material for prehistoric toolmaking. It is so hard it can scratch glass.
Field identification & locations
Identify in the field by its smooth, waxy surface and the way it breaks into sharp, curved flakes. Commonly found in limestone outcrops or as rounded pebbles in riverbeds.
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