sedimentary

Chert (Nodule)

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 on Mohs scale; Color: Gray, green, or brown (often waxy or dull); Luster: Vitreous to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, green, or brown (often waxy or dull)
Luster: Vitreous to waxy

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds. Geological age ranges from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (arrowheads and scrapers) due to its sharp edges. Modernly used as a decorative stone, in construction aggregate, and as a low-cost gemstone for tumbling.

Geological facts

Chert is so hard it can strike sparks against steel, leading to its frequent historical use as flint. It is chemically identical to quartz but contains microscopically small crystals that give it its opaque, waxy appearance.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breaks. Often found in riverbeds or eroding out of limestone cliffs. Popular for amateur collectors due to its durability.

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