sedimentary

Chert (Iron-stained)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, and grey with iron oxide staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, brown, and grey with iron oxide staining
Luster: Waxy to dull

Identified May 29, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, brown, and grey with iron oxide staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed from the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolaria) or from chemical precipitation from silica-rich fluids in low-energy sedimentary environments.

Uses & applications

Historically used for making stone tools (arrowheads). Currently used as aggregate in road construction, as a decorative garden stone, and in the manufacturing of glass and ceramics.

Geological facts

Chert is so hard it can create sparks when struck against steel. It is one of the most durable sedimentary rocks and often remains as 'float' after surrounding limestone has weathered away.

Field identification & locations

Identify by its waxy luster, extreme hardness (it will scratch glass), and edges that break into sharp, curved surfaces. Found worldwide in sedimentary basins, often as nodules or bedded layers in limestone.