sedimentary
Chert (Basanite variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale; Color: Dark grey to black with light quartz veining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Dark grey to black with light quartz veining
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 on Mohs scale; Color: Dark grey to black with light quartz veining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians) or chemical precipitation in marine environments. Often found as nodules or stream-rounded pebbles.
Uses & applications
Historically used for stone tools (lithics) due to sharp edges when fractured. Currently used for landscaping, road base, and as decorative river stones.
Geological facts
Black chert is often called Lydian stone or Basanite and was historically used as a 'touchstone' to test the purity of gold and silver by observing the color of the streak left behind.
Field identification & locations
Identify in the field by its hardness (scratches glass), smooth waxy texture, and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds or coastal deposits.
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