sedimentary

Chert (specifically Flint or Gray Chalcedony)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale. Color: Grayish-lavender to white. Luster: Waxy to dull. Crystal Structure: Cryptocrystalline (microscopic). Cleavage: None (notable for conchoidal, shell-like fracture). Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Grayish-lavender to white
Luster: Waxy to dull

Identified March 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical replacement of limestone/chalk by silica-rich groundwater. Found throughout geological history from the Precambrian to recent periods.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to sharp edges; industrially used as an abrasive, and sometimes used in lapidary work for jewelry or tumbled stones.

Geological facts

Chert was one of the most vital materials for early humans because its conchoidal fracture allows it to be 'knapped' into razor-sharp edges. It is also famous for its use in 'flint and steel' to create sparks for fire.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its waxy feel, and its curved, glass-like fracture surfaces. It is found globally, often occurring as nodules within sedimentary rock layers like limestone.

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