sedimentary
Chert (specifically Black Chert or Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Dark grey to black with white weathered rind; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Dark grey to black with white weathered rind
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark grey to black with white weathered rind; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids. Commonly found as nodules in limestone or chalk beds, often dating from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as road gravel, in concrete, and as a component in glassmaking. Polished specimens are used in jewelry and lapidary art.
Geological facts
Chert was the primary material for the 'Stone Age' because its conchoidal fracture allows it to be knapped into extremely sharp blades. It produces sparks when struck against steel, leading to its extensive use in flintlock firearms.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), smooth waxy feel, and curved 'shell-like' fractures. Commonly found in riverbeds or near limestone outcrops. Collectors look for pieces with unique banding or translucent edges.
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