sedimentary

Chert (specifically Black Chert or Flint)

Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Dark grey to black; Luster: Dull to waxy; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Dark grey to black
Luster: Dull to waxy

Identified April 11, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Dark grey to black; Luster: Dull to waxy; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation in marine environments. Often found as nodules or layers within limestone or chalk beds.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used as a road aggregate, in jewelry (when decorative), and for abrasive purposes.

Geological facts

Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, which made it the primary material for flintlock firearms and ancient survival. It breaks with a conchoidal fracture, creating edges sharper than a surgeon's scalpel.

Field identification & locations

Identify in the field by its smooth, waxy texture and shell-like (conchoidal) curved breaks. It will easily scratch glass. Commonly found in limestone outcrops or as river-worn cobbles.