sedimentary

Chert (Commonly known as Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Brown, tan, and gray waxy tones; Luster: Dull to sub-vitreous (waxy); Crystal Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Brown, tan, and gray waxy tones
Luster: Dull to sub-vitreous (waxy)

Identified February 22, 2026More sedimentary →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Forms primarily through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in sedimentary environments. Common in limestone or chalk beds, found in strata ranging from Precambrian to recent geological ages.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used as a road aggregate, in jewelry as semi-precious gemstones (jaspers), and in some glass-making or abrasive industrial processes.

Geological facts

Flint is a variety of chert that has been vital to human evolution, providing the primary material for spearheads and arrowheads for millennia. It produces sparks when struck against steel due to the hardness causing minute particles of metal to ignite.

Field identification & locations

Identify by its characteristic conchoidal (shell-like) fracture and sharp edges. It will scratch glass easily. Commonly found in river beds, gravel pits, or embedded as nodules within limestone and chalk outcroppings.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary