sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs); Color: Gray to off-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs)
Color: Gray to off-white
Luster: Waxy to dull

Identified July 12, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs); Color: Gray to off-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed via the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine environments, often replacing calcium carbonate in limestone or chalk. Commonly found in sedimentary deposits from the Paleozoic to Mesozoic eras.

Uses & applications

Historically used for stone tools (knapping) and fire-starting; currently used in construction aggregate, road surfacing, and as a raw material for ceramics and glass.

Geological facts

Chert is so hard it can scratch glass and steel. High-quality varieties that occur in chalk are specifically referred to as flint. It has been used by humans for millions of years to create sharp-edged tools like arrowheads and knives.

Field identification & locations

Identify by its smooth, waxy texture and distinctive shell-like (conchoidal) fracture patterns when broken. Commonly found in river beds, beaches, and eroded limestone outcrops.