sedimentary
Chert (Gravel specimen)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or white with waxy luster; Crystal Structure: Trigonal (microscopic fibers); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or white with waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or white with waxy luster; Crystal Structure: Trigonal (microscopic fibers); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within limestone/chalk beds. These specimens can range from a few hundred thousand to billions of years old.
Uses & applications
Commonly used in construction as road aggregate or concrete filler. Historically used for stone tools (arrowheads). In modern times, polished specimens are used for aquarium gravel or landscaping.
Geological facts
Chert is so hard it can create sparks when struck against steel. It was one of the most important materials for early humans to create survival tools due to its predictable breaking pattern (conchoidal fracture).
Field identification & locations
Identify in the field by its extreme hardness (will scratch glass easily) and its smooth, shell-like fracture surfaces. It feels much smoother than surrounding limestone or sandstone. Commonly found in riverbeds and gravel pits.
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