sedimentary

Chert / Chalcedony

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs), Color: Gray, tan, brown, Luster: Waxy to vitreous, Crystal structure: Microcrystalline/Cryptocrystalline, No cleavage, Conchoidal fracture

Hardness: 6

Identified June 18, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs), Color: Gray, tan, brown, Luster: Waxy to vitreous, Crystal structure: Microcrystalline/Cryptocrystalline, No cleavage, Conchoidal fracture

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms) or chemical precipitation in marine environments, often replacing calcium carbonate in limestone.

Uses & applications

Historical use for stone tools (lithics), modern use in landscaping, construction aggregate, and occasionally as semi-precious gemstones or lapidary rough.

Geological facts

Chert was one of the most important minerals for early humans because its conchoidal fracture allowed for the creation of razor-sharp edges on arrows and spearheads.

Field identification & locations

Identify by its waxy texture, hardness (it will scratch glass), and edges that break like thick glass or obsidian. Commonly found in riverbeds or as nodules within limestone outcrops.