Sedimentary rock

Chert (Flint)

SiO2 (predominantly microcrystalline or cryptocrystalline quartz)

Hardness: 6.5-7 on Mohs scale. Color: Highly variable; can be gray, brown, black, white, red, green. Luster: Dull to waxy. Crystal structure: Cryptocrystalline (crystals too fine to be seen without high magnification). Cleavage: None; exhibits conchoidal fracture. Specific Gravity: 2.58-2.64.

Hardness: 6
Color: Highly variable
Luster: Dull to waxy

Identified October 4, 2025More sedimentary rock →

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Physical properties

Formation & geological history

Chert forms from the accumulation of silica-rich skeletal remains of microscopic marine organisms (diatoms, radiolarians, spongiae spicules) that settle on the seafloor, or through the replacement of other sediments (like limestone) by silica-rich fluids. It often forms as nodules or distinct beds within limestone or chalk formations. Geological age varies widely depending on the formation, ranging from Precambrian to Cenozoic.

Uses & applications

Historically, flint was crucial for making tools (arrowheads, knives, scrapers) due to its sharp edges and conchoidal fracture. It was also used for starting fires (striking against steel). In modern times, chert is sometimes used as an aggregate in construction, as a filler, and occasionally for decorative purposes.

Geological facts

Flint is a dark, usually black or dark gray, variety of chert. The term 'chert' is broader and encompasses all colors and forms of this silica-rich rock. The ability of chert and flint to produce extremely sharp edges when fractured was revolutionary for early human technology. The White Cliffs of Dover, though primarily chalk, contain abundant flint nodules.

Field identification & locations

To identify chert in the field, look for its characteristic conchoidal (shell-like) fracture, which produces very sharp edges, and its hardness (it will scratch glass and steel). It often occurs as nodules in limestone or as distinct layers. Its color is highly variable, but its fine-grained texture and hardness are key indicators. Commonly found worldwide in sedimentary basins, particularly associated with limestones and chalks. For collectors, well-formed nodules, ancient tools, or cherts with interesting colors or patterns are sought after.

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