sedimentary

Chert (Nodule)

Chert (SiO2)

Hardness: 7 (Mohs scale); Color: Dark brown to black with a weathered 'desert varnish' or patina; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline (composed of fibrous chalcedony); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7.

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

Identified March 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. This specimen displays a smoothed, wind-abraded exterior common in arid environments (ventifact).

Uses & applications

Historically used for stone tools (flintknapping) due to its sharp edges; modernly used as a decorative landscape stone, in road construction, and occasionally for lapidary work.

Geological facts

Darker varieties of chert are often called 'flint.' This specific piece shows a distinctive conchoidal (shell-like) fracture pattern on the broken surface, which is the same property that made it the primary material for arrowheads and scrapers in the Stone Age.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its smooth waxy feel, and the way it breaks into sharp, curved flakes. Commonly found in riverbeds, desert pavements, and as nodules protruding from limestone outcrops.