sedimentary
Chert (Flint variety)
Chert (Microcrystalline or Cryptocrystalline Quartz), SiO2
Hardness: 7 (Mohs scale). Color: Typically tan, brown, or grey with a dull to waxy luster. Crystal Structure: Trigonal (microcrystalline). Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Typically tan, brown, or grey with a dull to waxy luster
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Physical properties
Hardness: 7 (Mohs scale). Color: Typically tan, brown, or grey with a dull to waxy luster. Crystal Structure: Trigonal (microcrystalline). Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. It often occurs as nodules or layers within limestone or chalk beds. Geological age ranges from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools and weapons (arrowheads) due to its sharp edges. Modern uses include road surfacing, grindstones, and as a raw material for ceramics and glass. High-quality colorful varieties are used in jewelry as 'jasper'.
Geological facts
Chert was one of the first minerals exploited by early humans. It produces sparks when struck against steel, which made it essential for early fire-starting and flintlock firearms. Its conchoidal fracture creates edges sharper than a surgeon's steel scalpel.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, shell-like (conchoidal) fracture pattern. Found globally in sedimentary basins. Collectors should look for nodules in limestone outcrops or eroded fragments in riverbeds.
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