sedimentary

Chert (variously known as Flint)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, white, and translucent; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, tan, white, and translucent
Luster: Dull to waxy

Identified February 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or by chemical precipitation in groundwater replacing limestone or chalk. Commonly found in nodular form within carbonate rocks.

Uses & applications

Historically used for making stone tools and arrowheads due to its sharp conchoidal edges. Currently used as road surfacing material, in concrete aggregate, and occasionally as an ornamental gemstone when polished.

Geological facts

Flint was one of the first minerals ever 'mined' by prehistoric humans. It can create sparks when struck against iron or steel, which led to its use in flintlock firearms and early fire-starting kits.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), waxy feel, and smooth, curved shell-like fractures. It is commonly found in riverbeds, glacial till, or embedded in limestone cliffs. Collectors look for unique banding or fossils trapped within.

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Brecciated Chert (Microcrystalline Silica)

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