sedimentary

Chert (Iron-rich or Slate-like variant)

Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 Mohs; Color: Dark grey to charcoal black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None/Conchoidal fracture; SG: 2.65

Hardness: 7 Mohs
Color: Dark grey to charcoal black
Luster: Waxy to dull

Identified June 28, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Dark grey to charcoal black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None/Conchoidal fracture; SG: 2.65

Formation & geological history

Formed by the accumulation of siliceous remains of marine organisms or chemical precipitation from hydrothermal fluids in sedimentary basins. Often found in nodules or bedded layers.

Uses & applications

Historically used for stone tools (knapping) due to its sharp edges. Modernly used in construction aggregate, road base, and as decorative gravel.

Geological facts

The black color in this specimen is likely due to high organic carbon content or inclusions of manganese/iron oxides. Chert is so hard it can strike sparks from steel, which led to its historical use as 'flint'.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its curved, glass-like (conchoidal) fracture edges. Commonly found in riverbeds or limestone outcrops.