sedimentary
Chert (Flint)
Cryptocrystalline Silica (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-orange exterior with waxy grey/blue interior, Luster: Waxy to vitreous, Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-orange exterior with waxy grey/blue interior, Luster: Waxy to vitreous, Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed as nodules in limestone or chalk beds (Upper Cretaceous period) through the replacement of calcium carbonate by silica-rich fluids. The orange patina is caused by iron oxidation from long-term exposure in gravel beds.
Uses & applications
Historically used for stone tools and starting fires; currently used as construction aggregate, road base, and landscape stone.
Geological facts
This specimen is highly characteristic of 'Essex Flint.' The location 51.8°N, 1.1°E is in North Essex near Clacton-on-Sea, a region famous for Pleistocene gravel deposits rich in these water-worn flint nodules.
Field identification & locations
Identified by the distinct conchoidal (shell-like) fracture visible in the chipped areas and the rough, weathered cortex. Common across the United Kingdom's southern and eastern coasts.
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