sedimentary

Chert (River Pebble)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Grayish-tan; Luster: Waxy to dull; Crystal structure: Microcrystalline (Cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Grayish-tan
Luster: Waxy to dull

Identified July 8, 2026More sedimentary →

Explore Chert (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-tan; Luster: Waxy to dull; Crystal structure: Microcrystalline (Cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed as nodules or layered deposits in sedimentary rocks (typically limestone or chalk) through the accumulation of silica-rich organic remains or chemical precipitation. This specific specimen has been smoothed by water transport in a river or glacial environment.

Uses & applications

Mostly used as gravel, road base, or construction aggregate. Historically used for making stone tools (arrowheads/scrapers) due to its sharp conchoidal fracture.

Geological facts

Chert is a tough, durable rock that often survives long periods of erosion. Flint is a common variety of chert. When two pieces are struck together, they can create sparks, which led to their historical use in starting fires.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, waxy texture. Frequently found in stream beds, gravel pits, and eroding out of limestone cliffs.