sedimentary

Chert

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, beige, or yellow-brown (due to iron oxides); Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Hardness: 7 on Mohs scale
Color: Tan, beige, or yellow-brown (due to iron oxides)
Luster: Waxy to dull

Identified June 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation of microcrystalline quartz or silica-rich organic remains (like diatoms or radiolarians) in marine or freshwater environments. This piece is likely a weathered nodule from a limestone or dolostone host rock.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when broken. Today, it is used in road construction, as an abrasive, and sometimes for lapidary work.

Geological facts

Chert is so hard it can scratch steel. It was one of the most important minerals for early humans to create fire-starting kits and hunting tools.

Field identification & locations

Identify in the field by its extreme hardness, smooth waxy feel on fresh breaks, and distinctive curved 'conchoidal' fractures. It is commonly found in stream beds and eroding out of sedimentary cliffs.