sedimentary
Chert (specifically Flint)
Chert (Microcrystalline or Cryptocrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Brown, tan, grey, and white banding; Luster: Waxy to vitreous (glassy) on fresh breaks; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.65
- Hardness
- 7 on Mohs scale
- Color
- Brown, tan, grey, and white banding
- Luster
- Waxy to vitreous (glassy) on fresh breaks
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, tan, grey, and white banding; Luster: Waxy to vitreous (glassy) on fresh breaks; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in sedimentary basins. Often found as nodules within limestone or chalk beds. Geological age varies from Precambrian to Recent.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to sharp conchoidal edges and as fire-starters. Modernly used as a decorative stone, in lapidary work for jewelry (cabochons), and as a component in road fill or concrete aggregate.
Geological facts
Flint was one of the most important minerals for early human survival, used for both weaponry and starting fires. When struck against steel, it creates sparks. The sharp edges of a fresh break can be thinner than a surgical steel scalpel.
Field identification & locations
Identify by its 'shell-like' (conchoidal) fracture pattern and its ability to scratch glass. Look for it in areas with limestone outcroppings or as smoothed nodules in riverbeds. These specific specimens show a classic 'rind' or weathered cortex on the outside.
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