sedimentary

Chert (and Chert Breccia)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs); Color: Dark grey, charcoal, black; Luster: Waxy to vitreous; Crystal structure: Microcrystalline/cryptocrystalline; Conchoidal fracture; Specific gravity: 2.6-2.7

Hardness: 6
Color: Dark grey, charcoal, black
Luster: Waxy to vitreous

Identified July 1, 2026More sedimentary →

Explore Chert (and Chert Breccia) 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: Dark grey, charcoal, black; Luster: Waxy to vitreous; Crystal structure: Microcrystalline/cryptocrystalline; Conchoidal fracture; Specific gravity: 2.6-2.7

Formation & geological history

Formed by the accumulation of siliceous remains of marine organisms (diatoms and radiolarians) or by chemical precipitation. This specimen appears brecciated, meaning it was broken into fragments and re-cemented by later mineral growth.

Uses & applications

Historically used for stone tools (arrowheads). Today used as decorative stone, construction aggregate, and in some gemstone jewelry.

Geological facts

Chert is so hard it can create sparks when struck against steel. It is chemically identical to quartz but has a much smaller grain size and contains impurities that create various colors.

Field identification & locations

Identify by its waxy luster, extreme hardness (scratches glass), and shell-like (conchoidal) fracture pattern. Commonly found in limestone and dolomite formations worldwide.