sedimentary

Chert (and/or River Pebble)

Microcrystalline quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Grayish-blue or dark gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Grayish-blue or dark gray
Luster: Waxy to dull

Identified May 9, 2026More sedimentary →

Explore Chert (and/or River Pebble) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 on Mohs scale; Color: Grayish-blue or dark gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine or freshwater environments. The rounded shape indicates it was smoothed by water erosion over a long period.

Uses & applications

Historically used for stone tools and starting fires (flint). Currently used in construction aggregate, landscaping, and sometimes as a decorative river stone.

Geological facts

Chert is so hard it can scratch glass. It was one of the first minerals used by early humans to create sharp cutting tools due to its predictable breaking pattern (conchoidal fracture).