sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 Mohs scale; Color: Dark brown to black (flint); Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 Mohs scale
Color: Dark brown to black (flint)
Luster: Dull to waxy

Identified June 19, 2026More sedimentary →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 Mohs scale; Color: Dark brown to black (flint); Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed via the accumulation of siliceous remains of organisms like diatoms and radiolarians on the seafloor, or through the chemical replacement of carbonate minerals by silica in groundwater.

Uses & applications

Historically used for stone tools like arrowheads and scrapers. Currently used in road construction, stone jewelry (when colorful), and as a starting material for silicon.

Geological facts

Chert is one of the most common materials used by prehistoric humans due to its ability to break with very sharp edges. Darker varieties are specifically often referred to as 'Flint'.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by steel) and curved, glass-like (conchoidal) fractures. Often found as nodules or layers in limestone or chalk beds.