sedimentary

Chert (specifically Flint or Common Chert)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Brown, gray, black, or tan; Luster: Waxy to dull; Crystal Structure: Microcrystalline (Cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Brown, gray, black, or tan
Luster: Waxy to dull

Identified July 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or sponge spicules) or by the precipitation of silica from groundwater within limestone or chalk beds. Often found in nodes or lenses within sedimentary strata.

Uses & applications

Historically used for stone tools (knapping) and fire-starting (flint and steel). Currently used in road surfacing, as grinding media in ball mills, and occasionally in jewelry as cabochons.

Geological facts

Chert is so tough that it was the primary material for spearheads and arrowheads for thousands of years. It produces a spark when struck against steel due to its hardness and friction.

Field identification & locations

Identify in the field by its characteristic waxy texture and sharp, shell-like (conchoidal) edges when broken. It cannot be scratched by a steel knife. Commonly found in limestone 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

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