sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs scale; Color: typically gray, brown, or black; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8.

Hardness: 7 Mohs scale
Color: typically gray, brown, or black
Luster: waxy to dull

Identified June 18, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs scale; Color: typically gray, brown, or black; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8.

Formation & geological history

Formed via the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers); currently used in road construction, as a source for silica, and in lapidary work as a gemstone when colorful.

Geological facts

Chert is so hard it can strike sparks against steel, making it vital for early fire-starting tools and flintlock firearms. Its sharp edges when fractured led to its primary role in the Stone Age.

Field identification & locations

Identify by its characteristic conchoidal (shell-like) fracture, extreme hardness (it will scratch glass), and waxy texture. Commonly found in riverbeds or eroded from limestone outcrops.