sedimentary

Chert (with Chalky Cortex)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Dark grey to brown interior with a white, chalky exterior (cortex); Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Luster: Waxy to dull

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of silica-rich fluids within sedimentary rock layers, often chalk or limestone. As the surrounding limestone weathers away, the harder chert nodules remain. Many date back to the Cretaceous period (approx. 66-145 million years ago).

Uses & applications

Abrasives, construction aggregates, and historically used for stone tools (flintknapping) due to its sharp edges. Low-grade specimens are common decorative stones.

Geological facts

Chert is chemically identical to quartz but has a different microscopic structure. When found in chalk deposits in Europe, it is specifically referred to as 'Flint'. High-quality chert was the primary material for prehistoric arrowheads and spear points.

Field identification & locations

Identify in the field by its white outer 'skin' and smooth, glass-like interior fracture when broken. It will easily scratch glass. Common in riverbeds, coastal cliffs of England and France, and limestone regions of the United States.

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