sedimentary
Chert (Common Opal or Flint variant)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: White to off-white; Luster: Waxy to dull; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- White to off-white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: White to off-white; Luster: Waxy to dull; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed primarily in sedimentary environments through the deposition of silica-rich fluids within limestone or chalk beds, or from the accumulation of siliceous organisms like diatoms and radiolarians.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modernly used as a decorative stone, in construction aggregate, and occasionally in jewelry as tumbled stones.
Geological facts
Chert is essentially the same mineral as flint, but the name 'flint' is usually reserved for dark nodules found in chalk. It is highly resistant to chemical weathering and often remains as a lag deposit after the surrounding rock has eroded.
Field identification & locations
Identify in the field by its waxy luster, hardness (it will scratch glass), and its characteristic conchoidal (shell-like) fracture pattern. Commonly found in stream beds or within carbonate rock outcrops.
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