sedimentary

Chert (potentially Jasper/Common Opal mixture)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Variable (browns, tans, purple-grey); Luster: Dull to waxy; Crystal Structure: Cryptocrystalline (non-visible); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 6
Color: Variable (browns, tans, purple-grey)
Luster: Dull to waxy

Identified June 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms primarily through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids. Often found in seafloor deposits or as nodules in limestone/chalk.

Uses & applications

Historically used for stone tools (arrowheads). Currently used as an industrial abrasive, in road construction (aggregate), and as decorative landscaping stone. High-quality colorful pieces are used for lapidary work.

Geological facts

Because it breaks with a conchoidal fracture like glass, it creates extremely sharp edges; it was one of the most vital materials for prehistoric human survival.

Field identification & locations

Identify by its smooth, flint-like fracture and high hardness (it will scratch glass but cannot be scratched by a steel knife). Commonly found in sedimentary basins and riverbeds as rounded 'river rocks'.

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

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