sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs scale; Color: Dark grey, brown, or black; Luster: Dull to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs scale
Color: Dark grey, brown, or black
Luster: Dull to waxy

Identified July 1, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs scale; Color: Dark grey, brown, or black; Luster: Dull to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or by chemical precipitation from silica-rich groundwater within limestone or chalk beds. Commonly found in Paleozoic and Mesozoic sedimentary layers.

Uses & applications

Historically used for making stone tools (arrowheads, knives) and strike-a-lights for fire making. Modern uses include road surfacing materials, decorative landscaping, and occasionally jewelry (as jasper).

Geological facts

Because of its conchoidal fracture, chert breaks into pieces with very sharp edges, leading to its widespread use by prehistoric humans for hunting tools and weapons.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife), waxy luster, and distinctive shell-like (conchoidal) curved fractures. Common in limestone outcrops.