sedimentary
Chert (Basanite / Jasperoid)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs. Color: Dull grey, black, or olive. Luster: Waxy to dull. Crystal Structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Dull grey, black, or olive
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs. Color: Dull grey, black, or olive. Luster: Waxy to dull. Crystal Structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary environments. Many river-tumbled specimens are from the Cretaceous or Jurassic periods.
Uses & applications
Historically used for stone tools/arrowheads. Modernly used in river rock landscaping, construction aggregate, and as a low-cost gemstone (lapidary tumbling).
Geological facts
Because of its high silica content and microcrystalline structure, it breaks with sharp edges similar to glass, making it the primary material for prehistoric survival tools.
Field identification & locations
Identify by its hardness (cannot be scratched by a steel pocket knife) and smooth, rounded surface from water erosion. Commonly found in riverbeds and glacial till throughout North America and Europe.
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