sedimentary

Chert (and Flint)

Silicon dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, brown, or reddish due to impurities; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, tan, brown, or reddish due to impurities
Luster: Waxy to dull

Identified May 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or the chemical replacement of limestone/dolomite by silica-rich fluids. Can be found in bedding planes or as nodules within carbonate rocks.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used in road construction, as architectural stone, and occasionally for lapidary work or jewelry when colorful (Jasper).

Geological facts

Chert has been used by humans for over 2 million years to make arrowheads and scrapers. Because it breaks with a conchoidal fracture, it creates edges sharper than a steel scalpel.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its shell-like (conchoidal) fracture surfaces, and its smooth, waxy texture. Commonly found in limestone outcrops or as river cobbles.