sedimentary
Chert (often called Flint for dark variants)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Grey to black (in this case); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grey to black (in this case)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Grey to black (in this case); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation in marine or freshwater environments. It often forms as nodules or bedded deposits within limestone or chalk.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges when broken. Today used in construction aggregate, road surfacing, and as a strike-stone for fire-starting. Higher-grade varieties (Jasper/Agate) are used in jewelry.
Geological facts
Chert was one of the most important materials for early humans because of its 'conchoidal' (shell-like) fracture pattern, which creates edges sharper than a surgical scalpel. It is also famous for emitting sparks when struck against steel.
Field identification & locations
Identify in the field by its extreme hardness (it will scratch glass) and its smooth, curved fracture surfaces. This specific piece appears to be a small, water-worn or weathered nodule. It is found globally in sedimentary basins.
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