sedimentary

Chert / Flint

Cryptocrystalline quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: typically dark gray, black, or brown with a white/tan outer cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic quartz); Cleavage: none, exhibits distinct conchoidal fracture; Specific gravity: 2.65.

Hardness: 6
Luster: waxy to dull

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of marine organisms (diatoms, radiolarians) or the chemical replacement of limestone/chalk by silica-rich fluids. Commonly found as nodules or layers within sedimentary rock formations ranging from the Precambrian to Cenozoic eras.

Uses & applications

Historically used for stone tools (knapping) and making fire (striking steel). Modern uses include road surfacing, as an abrasive, and sometimes in jewelry (as a variety of chalcedony).

Geological facts

Flint is technically a variety of chert found in chalk or marly limestone. It played a pivotal role in human evolution, as its sharp edges and ability to create sparks allowed for the development of weaponry and the control of fire.

Field identification & locations

Identify in the field by its smooth, shell-like fracture surfaces (conchoidal), its ability to scratch glass, and the presence of a chalky white outer rind or 'cortex.' It is commonly found in beach shingles or embedded in limestone cliffs. Collectors should look for unique banding or archaeological knapped artifacts.