sedimentary

Chert Nodule

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, white with a chalky outer cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.

Hardness: 7 (Mohs scale)
Color: Gray, tan, white with a chalky outer cortex
Luster: Waxy to dull

Identified January 5, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily in marine sedimentary environments (such as limestone or chalk beds) through the replacement of carbonate material by silica-rich fluids. It often accumulates in irregular nodular forms or layers.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modernly used in construction aggregate, road surfacing, and as a low-cost gemstone (jasper/agate varieties).

Geological facts

Chert is essentially the same material as flint; the term 'flint' is usually reserved for dark gray or black chert found in chalk. It is high in silica and can preserve microfossils perfectly.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture, extreme hardness (it will scratch glass), and its frequent occurrence as irregular lumps within softer limestone or chalk outcrops.