sedimentary

Chert (Chalcedony)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Tan, brown, grey, or yellowish; Luster: Waxy to vitreous; Crystal structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 6
Color: Tan, brown, grey, or yellowish
Luster: Waxy to vitreous

Identified June 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed as a chemical precipitate or biochemical deposit, typically from the accumulation of siliceous oozes at the bottom of the ocean or through the replacement of limestone by silica-rich fluids.

Uses & applications

Historically used for stone tools (arrowheads); currently used as a semi-precious stone in jewelry, as high-quality aggregate in construction, and for precision laboratory grinding equipment.

Geological facts

Chert is so hard it can strike sparks against steel. Flint is a common variety of dark chert. It has been used by humans for hundreds of thousands of years to create sharp-edged tools and weapons.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and its shell-like (conchoidal) fracture pattern. It is common in limestone formations and riverbeds globally.