sedimentary

Limonite-stained Chert

Silicon Dioxide (SiO2) with Hydrated Iron Oxide (FeO(OH)·nH2O)

Hardness: 6.5–7 on Mohs scale (scratches glass); Color: Dark grey to charcoal with yellow-brown (ochre) iron staining; Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 6
Luster: Waxy to dull

Identified March 14, 2026More sedimentary →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains or chemical precipitation within sedimentary rock layers, often limestone. The yellow staining (limonite) occurs later through the oxidation and hydration of iron-rich minerals during weathering.

Uses & applications

Historically used for stone tools and starting fires (flint); currently used for construction aggregate, road surfacing, and as decorative landscape stone or lapidary material for polishing.

Geological facts

Chert is a incredibly durable rock that resists chemical weathering. Because of its sharp conchoidal fracture, it was the primary material used by prehistoric humans for arrowheads, spear points, and scrapers.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found in gravel pits, riverbeds, and as nodules within limestone or chalk across North America and Europe.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary