sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7.0 (Mohs); Color: tan, grey, brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.6-2.7.

Hardness: 7
Color: tan, grey, brown
Luster: waxy to dull

Identified July 9, 2026More sedimentary →

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Physical properties

Hardness: 7.0 (Mohs); Color: tan, grey, brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.6-2.7.

Formation & geological history

Formed by the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like diatoms or radiolarians) in marine environments. It typically occurs as nodules or layered deposits in limestone or chalk beds.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used in road surfacing, landscaping, and as a component in ceramics.

Geological facts

Chert is so hard it can scratch steel. When struck against steel, it creates sparks, which made it essential for flintlock firearms and early fire-making techniques. Some varieties contain micro-fossils visible under high magnification.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Found globally in sedimentary basins. Collectors often look for colorful 'jasper' variants or worked archaeological artifacts.