sedimentary

Chert (Iron-stained)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Brownish-yellow to tan due to iron oxide staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific Gravity: 2.6 – 2.65

Hardness: 7 on Mohs scale
Color: Brownish-yellow to tan due to iron oxide staining
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 (like diatoms or radiolarians) or chemical precipitation in marine or groundwater environments; commonly found in limestone or chalk beds, often from the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for stone tools (arrowheads and scrapers) due to its sharp edges; modern uses include construction aggregate, gemstones (when colorful), and a source of silicon.

Geological facts

Chert is so hard it can strike sparks against steel to start fires. It is nearly chemically inert and extremely resistant to weathering, which is why it is often found as loose stones in riverbeds long after the surrounding limestone has eroded.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel nail), conchoidal (shell-like) fracture patterns, and waxy texture. Commonly found in gravel pits, riverbeds, and as nodules in sedimentary rock formations.

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