sedimentary

Chert (Nodule)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: typically earthy tones like brown, gray, or tan with white veining; Luster: waxy to dull; Structure: microcrystalline (no visible grains); Cleavage: none, shows conchoidal (shell-like) fracture; Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Luster: waxy to dull

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from groundwater within limestone or chalk beds, or from the accumulation of siliceous organisms like diatoms and radiolarians. The geological age varies widely from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Currently used in road construction as aggregate, as a component in ceramics, and occasionally for lapidary work.

Geological facts

Chert is so hard it can spark when struck against steel, which made it essential for flintlock firearms. It is incredibly durable and resistant to chemical weathering, often outlasting the surrounding rock it formed in.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), smooth waxy feel, and curved 'scalloped' fracture surfaces. Commonly found in riverbeds, glacial till, and coastal cliffs near limestone deposits.

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

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Sandstone (with man-made markings)

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