sedimentary
Chert (often called Flint in dark varieties)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically earthy browns, greys, and blacks; Luster: waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: none, exhibits conchoidal fracture.
- Hardness
- 7 on Mohs scale
- Color
- typically earthy browns, greys, and blacks
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically earthy browns, greys, and blacks; Luster: waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: none, exhibits conchoidal fracture.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids. It occurs predominantly as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons due to its sharp edges; currently used in construction aggregate, as a gemstone (Jasper/Agate varieties), and in fire-starting kits.
Geological facts
Chert was one of the most important materials for early humans because its conchoidal fracture allows it to be 'knapped' into incredibly sharp blades. It is chemically very stable and resistant to weathering.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy feel, and smooth, curved shell-like fractures. It is commonly found in sedimentary basins and as 'river rock' in gravel deposits.
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