sedimentary

Chert (Jasper/Agate variety)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 Mohs; Color: Brown, tan, and greenish-grey banding; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65

Hardness: 6
Color: Brown, tan, and greenish-grey banding
Luster: Waxy to dull

Identified June 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich fluids in sedimentary environments or as nodules in limestone/chalk. Often involves the replacement of organic material or the deposition of silica from hydrothermal fluids.

Uses & applications

Commonly used in lapidary work for making cabochons, beads, and tumbled stones. Historically used for making sharp tools like arrowheads due to its conchoidal fracture.

Geological facts

Chert is so durable that it often survives as smooth pebbles in riverbeds and on beaches long after the surrounding rock has eroded away. Varieties with high iron content are typically called Jasper.

Field identification & locations

Identify in the field by its waxy texture, inability to be scratched by a steel knife, and smooth, shell-like (conchoidal) break patterns. Found globally in creek beds and gravel deposits.

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)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary