sedimentary
Chert (Flint variety)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Grey, blue-grey, or tan; Luster: Waxy to dull; Structure: Microcrystalline silicon dioxide; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grey, blue-grey, or tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Grey, blue-grey, or tan; Luster: Waxy to dull; Structure: Microcrystalline silicon dioxide; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in sedimentary basins. It often occurs as nodules in limestone or chalk beds. Geological age ranges from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp edges when fractured. Modern uses include road construction aggregate, fire-starting strikers (flint), and tumbling for lapidary use/jewelry.
Geological facts
Chert was one of the most important materials for early humans; it produces a spark when struck against steel. The 'conchoidal fracture' allows it to break into razor-sharp edges, a property shared with obsidian and glass.
Field identification & locations
Identify in the field by its waxy luster, hardness (it will scratch glass), and its distinctive curved (conchoidal) breaks. It is commonly found in riverbeds, along beaches, or embedded in limestone cliffs.
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