sedimentary

Chert (with Limonite staining)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray matrix with yellowish-brown surface staining; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray matrix with yellowish-brown surface staining
Luster: Waxy to dull

Identified March 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from seawater or by the replacement of calcium carbonate in limestone by silica. The yellow-brown staining is caused by 'limonite' (iron oxide/hydroxide) weathering on the surface. These specimens can range from Cambrian to Holocene in age.

Uses & applications

Historically used for stone tools and projectiles due to its sharp conchoidal fracture. Modernly used as road aggregate, architectural stone, and occasionally in lapidary work as a tumble-polished stone.

Geological facts

Chert is so hard that it can scratch steel. It was one of the most important minerals for early humans, alongside flint (a variety of chert), for making arrowheads and spear points.

Field identification & locations

Identify in the field by its extreme hardness, waxy texture on fresh surfaces, and smooth, shell-like (conchoidal) fractures. Often found in riverbeds as rounded nodules or within limestone outcrops. Common worldwide.

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