sedimentary

Chert / Flint

Chert (Silicon Dioxide, SiO2)

Hardness: 7 (Mohs scale). Color: Tan, brown, or grey with iron staining. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.

Hardness: 7 (Mohs scale)
Color: Tan, brown, or grey with iron staining
Luster: Waxy to dull

Identified May 3, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale). Color: Tan, brown, or grey with iron staining. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.

Formation & geological history

Formed through the accumulation of silica-rich organic remains or chemical precipitation within sedimentary rock layers, often limestone or chalk. Commonly found in nodules or bedded deposits throughout the geologic record.

Uses & applications

Historically used for making stone tools (arrowheads, knives) due to its conchoidal fracture. Modern uses include construction aggregates, road surfacing, and as a starting material for sparking fire with steel.

Geological facts

Chert was one of the most important minerals for early humans. Dark varieties are often referred to as flint. It is so hard that it can scratch glass and steel.

Field identification & locations

Identify by its extreme hardness, waxy texture on fresh surfaces, and characteristic smooth, curved (conchoidal) breaks. Often found in riverbeds or eroding out of limestone cliffs.