sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: typically brown, tan, gray, or reddish in this form; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture

Hardness: 7 on Mohs scale
Color: typically brown, tan, gray, or reddish in this form
Luster: waxy to dull

Identified July 7, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: typically brown, tan, gray, or reddish in this form; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture

Formation & geological history

Formed in marine or freshwater environments by the accumulation of siliceous remains of microscopic organisms like diatoms and radiolarians, or via chemical precipitation from silica-rich fluids. Often found as nodules within limestone or chalk beds.

Uses & applications

Historically used for stone tools and starting fires (flint); currently used in road construction, as decorative landscaping gravel, and occasionally for lapidary work.

Geological facts

Small, rounded fragments like this are common components of river gravel. Because it is harder than steel and breaks with sharp edges, it was the primary material for prehistoric arrowheads and scrapers.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and its tendency to break into smooth, curved 'shell-like' surfaces. Commonly found in stream beds and areas with exposed sedimentary rock layers.