sedimentary

Chert / Flint

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, and brown with oxidation; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Gray, tan, and brown with oxidation
Luster: Waxy to dull

Identified June 23, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray, tan, and brown with oxidation; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed through the precipitation of silica from groundwater or as a replacement of limestone/chalk by silica-rich fluids. Often found in nodules or bedded layers within limestone formations.

Uses & applications

Used historically for stone tools and fire-starting (flint); currently used in road construction aggregate and as a semi-precious gemstone when colorful (Jasper/Agate).

Geological facts

Chert is so tough and chemically stable that it often remains as pebbles in riverbeds long after the surrounding limestone has eroded away. Prehistoric humans highly valued it for its razor-sharp edges.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture patterns upon breaking and its ability to scratch glass. Commonly found in riverbeds, glacial till, and sedimentary outcrops.