sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, brown, or buff
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Forms primarily as nodules or layers in limestone or chalk through the accumulation of silica-rich organic remains such as radiolaria or diatoms. It can also form through chemical precipitation from silica-rich fluids.

Uses & applications

Historically used for stone tools and arrowheads; currently used in construction aggregate, road surfacing, and as a decorative stone in landscaping.

Geological facts

Finely grained chert was the primary material for the Stone Age tool industry due to its predictable conchoidal fracture pattern which creates razor-sharp edges.

Field identification & locations

Identified by its extreme hardness (scratches glass), waxy luster, and shell-like (conchoidal) fracture surfaces. Commonly found in sedimentary basins and along riverbeds or beaches where harder rocks accumulate.