sedimentary

Chert (Flint variety)

Chert (SiO2)

Hardness: 7 (Mohs scale); Color: tan, grey, beige; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8

Hardness: 7 (Mohs scale)
Color: tan, grey, beige
Luster: waxy to dull

Identified June 19, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: tan, grey, beige; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8

Formation & geological history

Formed as microcrystalline quartz nodules within sedimentary hosts like limestone or chalk. It often forms through the precipitation of silica-rich fluids in cavities or via biological replacement of carbonate. Common in Paleozoic and Mesozoic rock layers.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used as construction aggregate, road base, and occasionally in jewelry as or lapidary rough.

Geological facts

Flint is a variety of chert that occurs specifically in chalk or marl. It was one of the most important minerals for early human survival due to its ability to hold a sharp edge and create sparks when struck against steel.

Field identification & locations

Identify by its 'conchoidal' (shell-like) fracture edges, waxy feel, and extreme hardness (it will scratch glass). Commonly found in river beds or weathering out of limestone bluffs. Collectors should look for sharp edges and translucent thin sections.