sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 Mohs scale; Color: Greyish-blue, white, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs scale
Color: Greyish-blue, white, tan
Luster: Waxy to dull

Identified July 6, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs scale; Color: Greyish-blue, white, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed primarily by the accumulation and compaction of silica-rich organic remains such as radiolaria or diatoms on the seafloor, or through chemical precipitation in carbonate environments. Often found in Phanerozoic sedimentary sequences.

Uses & applications

Used historically for stone tools and weapons (flint-knapping), and modernly as a construction aggregate, for road surfacing, and as decorative gravel.

Geological facts

Chert has been used by humans for millions of years to create sharp-edged tools. A specific darker variety is famously known as Flint. It is incredibly chemically stable and resistant to weathering.

Field identification & locations

Identify in the field by its distinct waxy luster and smooth, curved shell-like (conchoidal) fractures. It will scratch glass but cannot be scratched by a steel knife. Common in limestone formations.