sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs scale; Color: Grey, blue-grey, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs scale
Color: Grey, blue-grey, tan
Luster: Waxy to dull

Identified July 17, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs scale; Color: Grey, blue-grey, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed by the accumulation of silica-rich biogenic remains (like radiolaria or diatoms) or by chemical precipitation from hydrothermal fluids in sedimentary basins. Often found as nodules in limestone or chalk beds.

Uses & applications

Used historically for stone tools (arrowheads, scrapers) due to its sharp edges; used in construction as road metal and in the manufacturing of glass and ceramics. Also popular for lapidary work.

Geological facts

Because it breaks with a conchoidal fracture, it creates edges sharper than a steel scalpel. It was one of the most important minerals for early humans for making fire and tools.

Field identification & locations

Identify in the field by its waxy luster, extreme hardness (scratches glass), and distinctive shell-like (conchoidal) curved fracture patterns. Commonly found in riverbeds or eroding out of limestone cliffs.