sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs scale; Color: Gray, brown, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.

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

Identified June 14, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 7 Mohs scale; Color: Gray, brown, or tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.

Formation & geological history

Formed primarily by the accumulation of siliceous remains of organisms like diatoms or radiolarians, or by inorganic precipitation in marine environments. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used in road aggregates, abrasives, and as architectural stone. High-quality colorful varieties are used in lapidary work.

Geological facts

Because it breaks with a conchoidal fracture like glass, it creates edges that are sharper than a surgical scalpel. It has been used by humans for over 2 million years to create tools.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel nail) and distinctive shell-like (conchoidal) fracture patterns. Frequently found in riverbeds or eroding out of limestone cliffs.