sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, white, and black; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.

Hardness: 7 on Mohs scale
Color: Gray, tan, white, and black
Luster: Waxy to dull

Identified July 6, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, tan, white, and black; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.

Formation & geological history

Formed primarily in deep sea environments where the accumulation of siliceous oozes from skeletons of microscopic organisms (like diatoms and radiolarians) undergoes diagenesis. It can also form through the replacement of limestone by silica-rich fluids.

Uses & applications

Used historically for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a road base material, in grit for abrasives, and as building stone.

Geological facts

Chert is so hard it can strike sparks against steel, which made it essential for flintlock firearms and early fire-starting kits. Its microcrystalline structure allows it to break like glass.

Field identification & locations

Identified by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found as nodules in limestone or as distinct bedded layers.