sedimentary
Chert (or Dark Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Dark brown to black, Luster: Waxy to dull, Structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.65
Identified More sedimentary →
Explore Chert (or Dark Flint) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 7 (Mohs), Color: Dark brown to black, Luster: Waxy to dull, Structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica in sedimentary environments, often replacing calcium carbonate or accumulating from the silica-rich remains of microscopic organisms (radiolarians/diatoms) on the ocean floor.
Uses & applications
Historically used for making stone tools due to its sharp edges; modernly used as a decorative stone, in construction aggregate, and occasionally in jewelry as cabochons.
Geological facts
Chert is so hard it can scratch steel and was used by ancient humans to create sparks for fire and sharp edges for weaponry. Dark, high-quality chert is often referred specifically as flint.
Field identification & locations
Identify by its extreme hardness, waxy texture, and conchoidal (shell-like) fracture patterns. Frequently found in limestone beds as nodules or layers.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sedimentary Sandstone
Arenite (Sedimentary Rock)
sedimentary
Fossiliferous Concretion
Septarian Concretion containing Brachiopods/Crinoids
fossil
Shale
Shale (clastic sedimentary rock)
sedimentary
Brown Sandstone (River Rock)
Arenite
sedimentary
Shale
Argillaceous sedimentary rock
sedimentary