sedimentary
Chert (variant Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale. Color: Tan, beige, grey, or black. Luster: Dull to waxy. Structure: Non-crystalline/Microcrystalline. Cleavage: None (exhibits conchoidal fracture). It is very dense and will scratch glass effortlessly.
- Hardness
- 7 on the Mohs scale
- Color
- Tan, beige, grey, or black
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on the Mohs scale. Color: Tan, beige, grey, or black. Luster: Dull to waxy. Structure: Non-crystalline/Microcrystalline. Cleavage: None (exhibits conchoidal fracture). It is very dense and will scratch glass effortlessly.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the replacement of carbonate minerals in limestone. This specific specimen’s rounded edges suggest it was weathered and transported by water, often originating in the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in road construction, as an abrasive, and occasionally in jewelry (tumbled stones). Smaller, attractive pieces are popular with rockhounds.
Geological facts
Chert is chemically nearly identical to quartz but has a different internal structure. When struck against steel, it can produce sparks, which is why it was historically used in flintlock firearms and for starting fires.
Field identification & locations
Identify in the field by its smooth, waxy feel and the way it breaks into curved, shell-like (conchoidal) shards with razor-sharp edges. It is commonly found in stream beds, gravel pits, or as nodules within limestone outcrops. It often feels heavier than it looks due to its density.
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