sedimentary

Chert (variant Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, brown with banding; Luster: Waxy to dull; Crystal structure: Microcrystalline (Cryptocrystalline) trigonal; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, tan, brown with banding
Luster: Waxy to dull

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica from water, often in marine environments where silica-rich fluids replace calcium carbonate in limestone or chalk beds. It typically dates from various periods ranging from Cambrian to Cretaceous.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp conchoidal fracture. Modern uses include road surfacing, jewelry (when polished), and as 'fire-starting' stones (flint and steel).

Geological facts

Chert has been used by humans for over 2 million years to create tools. It is so tough that it was the primary choice for 'flintlock' firearms during the 17th through 19th centuries.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found in limestone outcrops, river beds, or coastal chalk cliffs. Collectors should look for unique banding or color patterns.