sedimentary

Chert (Jasper/Flint variety)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: typically earthy tones of brown, tan, grey, and yellow due to iron oxide impurities; Luster: dull to waxy; Structure: cryptocrystalline with conchoidal (shell-like) fracture; Cleavage: none.

Hardness: 7 on Mohs scale
Luster: dull to waxy

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the chemical precipitation of silica in groundwater within limestone or chalk beds. Most specimens found today are hundreds of millions of years old.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; currently used for decorative stone, road surfacing, jewelry (when colorful), and as a fire-starting material (flint).

Geological facts

Chert has been used by humans for over two million years. Because of its extreme hardness and way it breaks into sharp edges, it was the primary material for arrowheads and spear tips in ancient cultures.

Field identification & locations

Identify in the field by its smooth, curved 'conchoidal' fracture patterns and its ability to scratch glass. It often occurs as nodules inside limestone. Common globally, notably in the Midwestern USA and UK chalk cliffs.

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