sedimentary

Chert (Chalcedony/Agate variety)

Silicon dioxide (SiO2)

Hardness: 7 (Mohs scale), Color: Grayish-blue, white, and tan with iron staining, Luster: Waxy to vitreous, Crystal structure: Trigonal (microcrystalline/cryptocrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7

Identified July 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation in sedimentary environments. Silica fills voids and replaces other minerals in host rocks over millions of years.

Uses & applications

Historically used for stone tools (knapping) due to its sharp fracture edges. Currently used as an ornamental stone, in turquoise-style jewelry, and as a component in construction aggregates.

Geological facts

Chert is so durable that it often survives long after its host rock has weathered away. The bluish/grayish translucent variety seen here is often called chalcedony. It can spark when struck against steel.

Field identification & locations

Identify by its waxy luster, smoothness on broken surfaces, and extreme hardness (it will scratch glass). Commonly found in limestone or chalk beds and along riverbeds as durable nodules.

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Sandstone or Siltstone

Arenite (if sandstone)

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Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Sandstone

Arenite (primarily SiO2)

sedimentary