sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 Mohs; Color: grey, brown, tan; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs
Color: grey, brown, tan
Luster: waxy to dull

Identified May 30, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: grey, brown, tan; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed by the precipitation of silica from groundwater within limestone or chalk beds, or from the accumulation of siliceous organisms like diatoms and radiolarians. Often dates from the Paleozoic to Mesozoic eras.

Uses & applications

Historically used for stone tools/arrowheads due to its sharp edges; used today in road construction, as jewelry (when patterned), and for fire-starting strikers.

Geological facts

Chert nodules are often mistaken for fossils or bones due to their organic shapes. They are extremely resistant to weathering and are often found in riverbeds long after their host limestone has eroded away.

Field identification & locations

Identify in the field by its smooth, waxy texture and shell-like (conchoidal) fractures where broken. It will scratch glass but cannot be scratched by a steel knife. Frequently found in limestone quarries and stream beds.