sedimentary
Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs, Color: Gray, tan, brown with a white/weathered cortex, Luster: Waxy to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 Mohs, Color: Gray, tan, brown with a white/weathered cortex, Luster: Waxy to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed as nodules or beds in sedimentary rocks like limestone or chalk when silica precipitates from groundwater or biological sources (like sponge spicules) during diagenesis.
Uses & applications
Historically used for stone tools and weapons (arrowheads); modernly used for decorative landscaping, road aggregate, and as a component in glass and ceramics.
Geological facts
Flint is a variety of chert that produced sparks when struck against steel, making it essential for fire-making and early firearms (flintlocks). It was the primary material for the Stone Age technology.
Field identification & locations
Identified by its sharp edges and conchoidal (shell-like) fracture patterns. Look for a smooth, glass-like interior covered by a rough, chalky outer rind (cortex) in gravel pits or beach cliffs.
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