sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Dark grey to black center with a light tan/white outer rind (cortex); Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Luster: Waxy to dull

Identified April 3, 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 within limestone or chalk beds. This specimen often forms as nodules during diagenesis in marine environments.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative stone, in construction aggregate, and as a starting material for some industrial abrasives.

Geological facts

Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, which is why a variety of chert known as flint was used in early firearms and fire-starting kits.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It often features a weathered outer 'skin' or cortex that is softer and lighter in color than the interior.