sedimentary

Chert (Nodule)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Variegated grays, browns, and whites; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline/Hexagonal; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Variegated grays, browns, and whites
Luster: Dull to waxy

Identified March 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through chemical precipitation from silica-rich fluids within sedimentary host rocks like limestone or chalk. Many nodules originate from the accumulation of silica-based skeletal remains of marine organisms such as sponges or radiolarians, often occurring in strata from the Paleozoic and Mesozoic eras.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modern uses include aggregate in construction, road surfacing, and as a durable material for stone polishing and tumbling for collectors.

Geological facts

Chert is so durable that it often survives as 'float' or loose nodules after the softer surrounding rock has eroded away. Historically, flint (a variety of chert) was essential for starting fires and for use in flintlock firearms.

Field identification & locations

Identify in the field by its characteristic smooth, shell-like (conchoidal) fracture and its ability to scratch glass easily. Look for it as rounded, lumpy nodules in limestone outcrops or as weathered river cobbles. Common globally, especially in the US Midwest and UK chalk downs.