sedimentary

Chert (variations include Flint)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Gray, black, brown, or tan; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.65.

Hardness: 7 on Mohs scale
Color: Gray, black, brown, or tan
Luster: Waxy to dull

Identified March 16, 2026More sedimentary →

Explore Chert (variations include Flint) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 7 on Mohs scale; Color: Gray, black, brown, or tan; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater within limestone or chalk beds. Often associated with Cretaceous and Jurassic geological periods.

Uses & applications

Used historically for stone tools (arrowheads, knives) due to its sharp edges. Modern uses include road surfacing, construction aggregates, and historically as the strike-element in flintlock firearms.

Geological facts

Chert was one of the most important minerals for early humans. When struck, it produces a sharp, glass-like edge and can create sparks when struck against steel, which made it vital for survival and tool-making for millennia.

Field identification & locations

Identify in the field by its very hard surface (it will scratch glass), its smooth, shell-like (conchoidal) fracture pattern, and often a white chalky outer 'rind' or cortex. Commonly found in nodular form within limestone outcrops.