sedimentary

Chert (Nodule)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Greyish-white (this specimen); Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 6
Color: Greyish-white (this specimen)
Luster: Dull to waxy

Identified April 12, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 on Mohs scale; Color: Greyish-white (this specimen); Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine environments, often replacing calcium carbonate in limestone. These specimens occur in various geological ages from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (flintknapping); currently used in road construction, as an abrasive, and occasionally for lapidary work.

Geological facts

Chert is so hard that it can scratch glass and steel. This specific specimen shows a 'nodular' or 'pebble' texture with a weathered cortex, characteristic of rocks tumbled in riverbeds or coastal environments.

Field identification & locations

Identify by its hardness (cannot be scratched by a knife) and its smooth, shell-like (conchoidal) fracture pattern. It is commonly found in limestone or chalk beds and along river banks.