sedimentary

Chert (Flint)

Chert (Cryptocrystalline Quartz, SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale); Color: Gray, brown, or blackish; Luster: Waxy to dull; Crystal structure: Hexagonal (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Gray, brown, or blackish
Luster: Waxy to dull

Identified April 6, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray, brown, or blackish; Luster: Waxy to dull; Crystal structure: Hexagonal (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed from the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in marine or groundwater environments. It typically forms within limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges; used in modern times for road surfacing, grinding media, and occasionally as fire-starting 'flint' in survival gear.

Geological facts

Chert is so hard it can scratch glass and steel. High-quality dark gray or black chert is often specifically called 'flint' and was the most important material for early human toolmaking and early firearms (flintlocks).

Field identification & locations

Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to spark when struck against steel. It is frequently found as nodules within limestone outcrops.