sedimentary

Chert (Flint)

Microcrystalline Silica (SiO2)

Hardness: 7 on Mohs scale; Color: typically dark brown, black, tan, or grey; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture (shell-like curves); Specific gravity: approx 2.65

Hardness: 7 on Mohs scale
Color: typically dark brown, black, tan, or grey
Luster: waxy to dull

Identified May 13, 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

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation from groundwater within limestone or chalk beds. Most specimens date from the Phanerozoic eon.

Uses & applications

Historically used for making stone tools (arrowheads, knives) and striking fire; currently used as a raw material in road construction, as an abrasive, and for primitive fire-starting kits.

Geological facts

Flint is the high-quality, dark-colored variety of chert. It was the most important material for human technology for over two million years. It creates sparks when struck against steel due to the hardness of the quartz particles.

Field identification & locations

Identify in the field by its very smooth, glass-like fracture surfaces and sharp edges. It will easily scratch glass. Commonly found as nodules or layers within sedimentary limestone or chalk outcrops and in river beds.