sedimentary

Chert / Flint

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown due to impurities; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, tan, white, or brown due to impurities
Luster: Waxy to dull

Identified March 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids. It often occurs as nodules or layers within limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and to start fires. Modern uses include abrasives, road surfacing, and as a component in glass and ceramics manufacture.

Geological facts

Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's scalpel. Flint is a specific dark variety of chert often found in chalk deposits.

Field identification & locations

Identify in the field by its smooth, shell-like fracture surfaces and ability to scratch glass. Commonly found in riverbeds, glacial tills, and embedded in sedimentary rock outcrops worldwide.