sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 Mohs; Color: Tan, brown, white, and gray mottling; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Hardness: 6
Color: Tan, brown, white, and gray mottling
Luster: Waxy to dull

Identified July 4, 2026More sedimentary →

Explore Chert (Flint) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 6.5-7 Mohs; Color: Tan, brown, white, and gray mottling; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Formation & geological history

Formed by the precipitation of silica from groundwater or the accumulation of siliceous skeletal remains of marine organisms. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons (arrowheads); currently used in road surfacing, as an abrasive, and by collectors for lapidary work.

Geological facts

Chert has a conchoidal fracture which means it breaks like glass, producing very sharp edges. This made it the premier material for prehistoric toolmaking.

Field identification & locations

Identify by its waxy texture, extreme hardness (scratches glass), and shell-like (conchoidal) fracture patterns. Frequently found in gravel pits or eroding out of limestone outcrops.