sedimentary

Chert (specifically Cryptocrystalline Quartz)

Silicon dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Brown, tan, and gray mottled; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Brown, tan, and gray mottled
Luster: Dull to waxy

Identified July 9, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Brown, tan, and gray mottled; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from hydrothermal fluids in sedimentary basins.

Uses & applications

Used historically for stone tools (flintknapping), as road aggregate in construction, and occasionally polished for lapidary use or aquarium stone.

Geological facts

Chert is extremely durable and chemically resistant. It was the primary material used by prehistoric humans to create arrowheads and scrapers due to its sharp, glass-like fracture edges.

Field identification & locations

Identify by its hardness (it will scratch glass) and its characteristic shell-like (conchoidal) fracture pattern. Commonly found in limestone and chalk deposits or as water-worn pebbles in riverbeds.