sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (hexagonal system); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, beige, or off-white
Luster: Waxy to dull

Identified June 26, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (hexagonal system); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed primarily through the precipitation of silica in marine environments, often replacing carbonate minerals or accumulating from the remains of silica-rich organisms like diatoms and radiolarians. Common in Paleozoic and Mesozoic strata.

Uses & applications

Historically used for making stone tools (arrowheads). Modern uses include road surfacing, decorative stone, and as a raw material in some ceramic glazes. It is highly valued as a specimen for teaching conchoidal fracture.

Geological facts

Chert is essentially a dense, microcrystalline version of quartz. When it is red, it is often called Jasper; when it is dark gray or black, it is called Flint. It is so tough that it was one of the first materials used by early humans for tools and fire-starting.

Field identification & locations

Identify in the field by its hardness (will scratch glass), waxy luster on fresh surfaces, and smooth, curved shell-like (conchoidal) fractures. Common in limestone outcrops and as river cobbles.