sedimentary

Chert Nodules (Limonitic Chert)

Cryptocrystalline Quartz (SiO2) with Hydrated Iron Oxides

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs); Color: Brown, tan, earthy yellow-brown; Luster: Dull to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Hardness: 6
Color: Brown, tan, earthy yellow-brown
Luster: Dull to waxy

Identified June 22, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs); Color: Brown, tan, earthy yellow-brown; Luster: Dull to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Formation & geological history

Formed via the accumulation of silica-rich fluids within sedimentary host rocks like limestone or chalk. The brown color is due to limonitic staining from iron oxides. Often found in Paleozoic to Cenozoic strata.

Uses & applications

Historically used for making stone tools (flint knapping) due to its sharp edges. Modernly used as road aggregate, architectural stone, and occasionally for tumbling as decorative pebbles.

Geological facts

This specimen appears to be 'Tripstow stone' or a similar iron-stained silicate nodule. Chert is so durable that it often remains behind after the surrounding host rock has eroded away.

Field identification & locations

Identified by its extreme hardness (will scratch glass), opaque nature, and dull, earthy exterior. Commonly found in creek beds and fields where sedimentary rock has weathered.