sedimentary

Chert (and Flint)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, brown, or black with some white weathering crust; Luster: Waxy to dull; Crystal Structure: Microcrystalline (Cryptocrystalline) Quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Gray, tan, brown, or black with some white weathering crust
Luster: Waxy to dull

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica from seawater or groundwater. It often forms as nodules or layered beds in limestone or chalk deposits. It can be found in strata ranging from Precambrian to Cenozoic ages.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp conchoidal edges; currently used as construction aggregate, road surfacing, or occasionally for lapidary work in jewelry.

Geological facts

Flint is a dark variety of chert. It was essential for early humans to create fire by striking it against steel or pyrite to produce sparks. It is extremely resistant to weathering, which is why it often remains as loose nodules in soil long after the surrounding limestone has eroded.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture pattern and its ability to scratch glass. It is commonly found in riverbeds, chalk cliffs, or limestone quarries. Collectors look for pieces with interesting color banding or fossil inclusions.