sedimentary
Chert (Grey Flint)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale), Color: Dull grey to dark charcoal, Luster: Waxy to vitreous, Crystal Structure: Cryptocrystalline, Cleavage: None (Conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dull grey to dark charcoal, Luster: Waxy to vitreous, Crystal Structure: Cryptocrystalline, Cleavage: None (Conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in ocean or lake water. Often found as nodules within limestone or shale beds spanning from the Precambrian to the present day.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Currently used in road construction as an aggregate, in gemstone tumbling for beginners, and for fire-starting in flint-and-steel kits.
Geological facts
Chert and flint were arguably the most important materials for early human technological development. Because it breaks with a conchoidal (shell-like) fracture, it produces edges that can be sharper than a modern steel scalpel.
Field identification & locations
Identify in the field by its extreme hardness (will scratch glass or steel) and its distinctive shell-like curved fractures. Commonly found in riverbeds, limestone outcrops, or as weathered pebbles on beaches. It often has a thin white 'cortex' on the exterior.
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