sedimentary

Chert (variety Gray or Dark Chert)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Gray to dark charcoal with light banding; Luster: Dull to waxy; Crystal Structure: Microcrystalline (Cryptocrystalline) quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray to dark charcoal with light banding
Luster: Dull to waxy

Identified June 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or via chemical precipitation from silica-rich fluids in limestone or chalk beds. Often Cretaceous to Paleozoic in age.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Modern uses include road surfacing, decorative gravel, and as a component in glass manufacturing.

Geological facts

Chert is so hard it can scratch steel and was widely used by prehistoric humans to create the first sophisticated tools and weapons. It is chemically identical to flint, though flint is technically a type of chert found specifically in chalk deposits.

Field identification & locations

Identify in the field by its hardness (cannot be scratched by a knife), smooth waxy feel on breaks, and sharp edges. It is common in sedimentary basins worldwide. When struck against steel, it can produce sparks, making it useful for fire-starting.

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Brecciated Chert (Microcrystalline Silica)

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