sedimentary

Chert (or Chalcedony)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale), Color: tan, yellow, cream with reddish staining, Luster: waxy to dull, Crystal structure: Trigonal (microcrystalline/cryptocrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7

Identified June 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily by the accumulation of siliceous remains of marine organisms or the chemical precipitation of silica in groundwater. Can be found in sedimentary beds or as replacement nodules in limestone. Most specimens range from hundreds of millions to billions of years old.

Uses & applications

Historically used for making stone tools (arrowheads). Today used in aquarium decor, landscaping, and as semi-precious gemstones (agate/jasper varieties) in jewelry making.

Geological facts

Chert has been used by humans for hundreds of thousands of years due to its conchoidal fracture which creates razor-sharp edges. It is chemically identical to quartz but has a different internal structure.

Field identification & locations

Identify by its hardness (it will scratch glass) and its curved, shell-like fracture surfaces. Often found in riverbeds, glacial tills, or embedded in limestone cliffs. It feels smoother and 'waxier' than common sandstone.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary