sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Dark grey, brownish-black, tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Dark grey, brownish-black, tan
Luster: Waxy to dull

Identified April 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical replacement of limestone/chalk by silica-rich fluids. It is found in sedimentary strata ranging from Precambrian to Cenozoic ages.

Uses & applications

Historically used for stone tools (knapping for arrowheads/scrapers). Modern use includes road surfacing, gemstones (when colorful), and fire-starting kits (flint).

Geological facts

Chert is so hard it will scratch steel and was used by ancient humans to produce the first complex tools. When struck against steel, it produces sparks due to the friction heating tiny particles of iron, which then ignite in the air.

Field identification & locations

Identify by its 'waxy' texture and smooth, curved (conchoidal) breakage patterns. Often found as nodules within limestone or as pebbles in riverbeds. These are common in many regions, especially near limestone outcrops.