sedimentary

Chert (Quartz)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Brown, tan, or grey; Luster: Waxy to dull; Crystal structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Brown, tan, or grey
Luster: Waxy to dull

Identified May 27, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Brown, tan, or grey; Luster: Waxy to dull; Crystal structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed from the accumulation of microcrystalline silica, often from the remains of siliceous organisms (like diatoms or radiolarians) or chemical precipitation from groundwater within sedimentary beds.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to conchoidal fracture. Today used in road construction, stone mulch, and as a low-cost gemstone for tumbling.

Geological facts

Chert was one of the first materials harvested by early humans to make tools. Varieties like flint are well-known for their ability to spark when struck against steel.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its smooth waxy feel when freshly broken, and its characteristic curved (conchoidal) fracture lines. Commonly found in limestone and chalk deposits.