sedimentary
Chert (specifically Flint or Jasper variant)
Microcrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: brown, tan, and cream; Luster: waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- brown, tan, and cream
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: brown, tan, and cream; Luster: waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of siliceous remains of microscopic organisms (diatoms, radiolarians) or by chemical precipitation in limestone or chalk beds. Most specimens occur in marine sedimentary layers from various geological ages.
Uses & applications
Historically used for stone tools and starting fires; currently used as abrasive media, road surfacing, and decorative lapidary material for jewelry (cabochons).
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture produces extremely sharp edges, ideal for arrowheads and scrapers. Some chert contains well-preserved microfossils.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its waxy feel, and its characteristic curved, shell-like (conchoidal) fracture pattern when broken. Commonly found in limestone outcrops or as river-worn cobbles.
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