sedimentary

Chert (Iron-stained)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale (scratches glass); Color: Mottled red, brown, white, and black due to iron and manganese oxides; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 6
Luster: Waxy to dull

Identified March 25, 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 biogenic remains (like diatoms or radiolarians) or the chemical replacement of limestone/dolomite by silica-rich fluids. Can be found in geological formations ranging from the Precambrian to the present.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; modernly used as a decorative stone, in construction aggregate, or for lapidary tumbling and jewelry.

Geological facts

Chert is so hard and dense that it was commonly used as 'flint' to start fires when struck against steel. The reddish hue in this specimen is specifically caused by hematite (iron oxide) impurities.

Field identification & locations

Identify in the field by its hardness (it should not be scratchable by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. Common in sedimentary basins, riverbeds, and as nodules within limestone outcrops.

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