sedimentary

Chert (and associated Sedimentary rocks)

Microcrystalline Silica (SiO2)

AI-generated identification · may be incorrect

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

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

Identified April 1, 2026More sedimentary →

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Physical properties

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

Formation & geological history

Formed through the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or by the chemical replacement of limestone/dolomite by silica-rich fluids. Usually found in Phanerozoic age strata.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to sharp edges; currently used for road base, decorative landscaping, and as a raw material for silicon-based products.

Geological facts

Chert is so hard it can scratch glass and steel. Flint is a high-quality variety of chert. It has been used by humans for millions of years to start fires and create weaponry.

Field identification & locations

Identify in the field by its very hard surface, waxy texture, and distinctive 'shell-like' shell-like (conchoidal) fractures. Often found in nodules within limestone layers or as river pebbles.