sedimentary

Chert (Jasper/Agate variety)

Microcrystalline Quartz (SiO2)

Hardness: 6.5–7 (Mohs scale), Color: Variegated greens and browns, Luster: Waxy to vitreous, Crystal structure: Trigonal/Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6–2.7

Hardness: 6

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains or chemical precipitation within sedimentary rock layers like limestone. It often forms in marine environments and can be of various geological ages ranging from Precambrian to Cenozoic.

Uses & applications

Primary use is for lapidary work (jewelry, cabochons) and as a collector's specimen. Historically used for making stone tools (arrowheads) due to its sharp edges when fractured.

Geological facts

Chert is so durable that it often remains behind as 'lag' after the surrounding limestone or chalk has eroded away. The green coloration often comes from chlorite or iron inclusions.

Field identification & locations

Identify in the field by its smooth, waxy surface and its ability to scratch glass. It produces a distinct curved (conchoidal) fracture pattern when broken. Commonly found in riverbeds or coastal rocky outcrops.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary