sedimentary

Chert (variously Flint or Jasper gravel)

Microcrystalline or cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale); Color: Tan, brown, or grey; Luster: Dull to waxy; Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Hardness: 7 (Mohs scale)
Color: Tan, brown, or grey
Luster: Dull to waxy

Identified March 27, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Tan, brown, or grey; Luster: Dull to waxy; Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Formation & geological history

Formed as nodules or layers in sedative rocks (mostly limestone or chalk) through the replacement of calcium carbonate by silica-rich fluids. Often millions of years old, typically dating from the Cretaceous to the Precambrian periods.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to conchoidal fracturing. Modernly used as construction aggregate, road base, and occasionally as semi-precious gemstones when colorful (Jasper).

Geological facts

Chert is so hard it can scratch glass and steel. When struck against steel, it produces sparks, which led to its historic use in 'flintlock' firearms. It is incredibly durable and resistant to chemical weathering.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch a knife blade. Commonly found in riverbeds, glacial till, and embedded in limestone outcrops. Collectors look for unique banding or translucent edges.