sedimentary

Chert (Nodule)

Microcrystalline quartz (SiO2)

Hardness: 7 on Mohs scale; Color: typically tan, gray, or brown; Luster: dull to waxy; Structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture; Specific gravity: approx. 2.65.

Hardness: 7 on Mohs scale
Color: typically tan, gray, or brown
Luster: dull to waxy

Identified June 15, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: typically tan, gray, or brown; Luster: dull to waxy; Structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture; Specific gravity: approx. 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments, often replacing carbonate rocks during diagenesis.

Uses & applications

Historically used for making stone tools (arrowheads/knives) due to its sharp fracture edges. Today used in gravel for construction, as a source for silica, and as a decorative rock in landscaping.

Geological facts

Chert is one of the most durable sedimentary rocks. It was the primary material for the 'Stone Age' because its conchoidal fracture creates edges sharper than a surgical scalpel.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by steel) and smooth, shell-like fracture surfaces. Often found as rounded nodules in limestone or chalk beds globally.