sedimentary

Chert (specifically Gray/Black Chert)

Chert (Microcrystalline or Cryptocrystalline Quartz, SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Gray, black, and tan (due to organic matter and iron oxides); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, black, and tan (due to organic matter and iron oxides)
Luster: Waxy to dull

Identified March 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the chemical precipitation of silica in marine or groundwater environments. This specimen appears weathered and rolled, suggesting it was transported by water.

Uses & applications

Used historically for stone tools (arrowheads/scrapers) due to its sharp edges; modern uses include road construction aggregate, landscaping, and as a refractory material in industrial processes.

Geological facts

Chert is so hard it can strike sparks against steel, making it a primary component in flintlock firearms and ancient fire-starting kits. The dark coloration is often caused by trace amounts of carbon or bridge-forming minerals.

Field identification & locations

Identify in the field by its very smooth, glass-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found in limestone or chalk beds as nodules. This specimen shows a thick weathering rind (cortex) on the exterior.