sedimentary

Chert nodules in Limestone

Microcrystalline Quartz (SiO2) in Calcium Carbonate (CaCO3) matrix

Hardness: 7 (chert) / 3 (limestone matrix). Color: Tan/brown chert with grey-white matrix. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: Conchoidal fracture in chert. Specific gravity: 2.6-2.7.

Hardness: 7 (chert) / 3 (limestone matrix)
Color: Tan/brown chert with grey-white matrix
Luster: Waxy to dull

Identified May 24, 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 via chemical precipitation of silica from seawater, often replacing carbonate material within limestone beds during diagenesis (post-depositional chemical changes). Typically found in marine sedimentary layers of various ages.

Uses & applications

Historically used for making stone tools (arrowheads). Today used as road aggregate, construction fill, and occasionally for lapidary work if the chert is colorful.

Geological facts

Chert is so hard it can strike sparks against steel. It was one of the most important minerals for prehistoric humans because it breaks with razor-sharp edges.

Field identification & locations

Identify in the field by looking for hard, waxy nodules that cannot be scratched by a steel knife, embedded within a softer, grey rock that reacts with weak acid. Commonly found in limestone outcrops or as weathered stones in creek beds.

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

Arenite

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary